OtoVFHSJTY  of  ILLINOIS  LMm 

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SOME  REACTIONS  INVOLVED  IN  SECONDARY 
COPPER  SULPHIDE  ENRICHMENT. 


E.  G.  Zies,  E.  T.  Allen  (Chemical  Study),  and  H.  E.  Merwin  (Microscopic 

Study) . 


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CORRECTIONS  TO  PAGE  REFERENCES. 

Page  428,  footnote  21,  correct  428  to  427. 

Page  429,  line  10,  correct  427  to  426. 

Page  436,  footnote  31,  correct  453  to  452. 

Page  447,  footnote  37,  correct  446  to  445. 

Page  447,  last  line  text,  correct  437  to  436. 

Page  448,  line  9,  correct  446  to  445. 

Page  452,  footnote  45,  correct  446  to  445. 

Page  455,  footnote  50,  correct  439  to  438. 

Page  465,  footnote  60,  correct  464  to  463. 

Page  465,  third  line  text,  correct  446  to  445. 

Page  468,  footnote  62,  correct  442  to  441. 

Page  469,  footnote  64,  correct  471  to  470. 

Page  469,  footnote  68,  correct  439  to  438. 

Page  474,  footnote  71,  correct  491  to  490. 

Page  474,  footnote  71,  correct  496  to  495. 

Page  477,  footnote  76,  correct  465  to  464. 

Page  479,  footnote  83,  correct  477  to  476. 

Page  497,  (footnote  100,  correct  428  to  427. 

Page  499, ' footnote  106,  correct  459  to  458. 


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[Reprinted  from  Economic  Geology,  Vol.  XI.,  No.  5,  July-August,  1916.  f 


SOME  REACTIONS  INVOLVED  IN  SECONDARY 
COPPER  SULPHIDE  ENRICHMENT. 

E.  G.  Zies,  E.  T.  Allen  (Chemical  Study),  and  H.  E.  Merwin  (Microscopic 

Study) . 

Syllabus. 

Page. 

I.  Introduction  40  8 

II.  General  Procedure  409 

III.  Apparatus  412 

IV.  Reactions  of  the  Enriching  Solutions  on  the  Individual  Sulphides  . . 420 

1.  Action  of  Cupric  Sulphate  on  Covellite  (CuS)  42a 

A.  The  Reaction  at  200°  421 

(a)  Experiments  on  Synthetic  Covellite  421 

( b ) Experiments  on  Natural  Covellite  424 

B.  The  Reaction  at  ioo°  426 

(7.  The  Reaction  at  250  to  40°  427 

2.  Action  of  Cuprous  Sulphate  on  Covellite 428 

3.  Action  of  Cupric  Sulphate  on  Chalcocite  (Cu2S)  430 

4.  Action  of  Cupric  Sulphate  on  Pyrite  (FeS2)  43 1 

A.  The  Reaction  at  200°  432’ 

(a)  The  Alteration  of  Pyrite  to  Chalcocite 432 

(1)  Methods  of  Analysis  of  Enrichment  Product 432 

(2)  Results  435 

( b ) The  Alteration  of  Pyrite  to  Cupric  and  Cuprous  Sul- 

phides   438 

(c)  The  Influence  of  Sulphuric  Acid  443 

(1)  Secondary  Reactions  446 

( d ) The  Influence  of  Ferrous  Sulphate  448 

B.  The  Reaction  at  ioo°  449 

C.  The  Reaction  at  40°  to  50°  451 

5.  Action  of  Cuprous  Sulphate  on  Pyrite  452 


408 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


6.  Alteration  of  Pyrite  into  the  Copper-iron  Sulphides  455 

7.  Action  of  Cupric  Sulphate  on  Pyrrhotite  455 

A.  The  Reaction  at  200°  455 

B.  The  Reaction  at  ioo°  . 460 

C.  The  Reaction  at  40°  461 

8.  Action  of  Cupric  Sulphate  on  Chalcopyrite  (CuFeS2)  463 

A.  The  Reaction  at  200°  464 

(a)  Action  of  Sulphuric  Action  on  Chalcopyrite 464 

( b ) Alteration  of  Chalcopyrite  to  Chalcocite  465 

( c ) Alteration  of  Chalcopyrite  to  Cupric  and  Cuprous  Sul- 

phide   467 

B.  The  Reaction  at  40°  469 

(a)  The  Influence  of  Sulphuric  Acid  471 

(b)  The  Influence  of  Ferrous  Sulphate  472 

C.  Discussion  474 

9.  Action  of  Cupric  Sulphate  on  Bornite  (Cu3FeS4)  475 

A.  Action  of  Sulphuric  Acid  on  Bornite 476 

B.  The  Reaction  at  200°  478 

(a)  Alteration  of  Bornite  to  the  Sulphides  of  Copper 478 

( b ) Discussion  of  Results  480 

C.  The  Reaction  at  100° 483 

D.  The  Reaction  at  Ordinary  Temperatures  484 

( a ) The  Influence  of  Sulphuric  Acid 486 

10.  Action  of  Cupric  Sulphate  on  Sphalerite  (ZnS)  486 

A.  The  Reaction  at  200°  486 

B.  The  Reaction  at  Ordinary  Temperatures 490 

(a)  The  Influence  of  Sulphuric  Acid 491 

11.  Action  of  Cupric  Sulphate  on  Galena  at  Ordinary  Temperatures 

(PbS)  493 

V.  Relative  Reactivities  of  the  Sulphides  towards  Cupric  Sulphate,  at  40°  497 

VI.  Summary  500 


I.  INTRODUCTION. 

In  recent  years  the  efforts  of  a number  of  investigators  have 
been  directed  toward  the  elucidation  of  the  chemistry  of  the  proc- 
ess known  in  geology  as  secondary  enrichment.1  This  process  as 
it  relates  to  copper  may  for  other  than  geological  readers  be 
simply  illustrated  by  what  happens  in  the  case  of  a copper-bear- 
ing  pyritic  ore.  When  such  an  ore  is  exposed  to  the  oxidizing 
influences  near  the  earth’s  surface,  copper  and  iron  sulphates 
and  sulphuric  acid  are  formed;  much  of  the  iron  is  concentrated 

1 See  F.  F.  Grout,  Econ.  Geol.,  VIII.,  p.  408,  for  bibliography  of  the  ex- 
perimental work  on  this  subject,  and  W.  H.  Emmons,  Bull.  U.  S.  Geol.  Survey, 
No.  529,  11-12,  and  J.  D.  Clark,  Bull.  Univ.  of  Mex.,  75,  p.  142,  for  extensive 
bibliographies  of  the  general  subject  of  secondary  enrichment. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4°9 


near  the  surface  as  hydrous  iron  oxide,  while  the  copper  sulphate, 
together  with  iron  sulphate  and  sulphuric  acid,  passes  downward 
in  solution.  The  copper  is  precipitated  by  the  influence  and  at 
the  expense  of  the  still  unoxidized  sulphides.  The  state  of  com- 
bination in  which  the  copper  is  deposited  is  believed  to  depend 
on  the  particular  sulphides  of  which  the  ore  is  composed,  as  well 
as  on  the  composition  of  the  enriching  solution. la 

In  this  paper  are  considered  some  of  the  reactions  involved 
when  the  sulphides  commonly  present  in  copper  ores  are  exposed 
to  the  action  of  the  enriching  solutions.  It  covers  only  one 
phase  of  the  broad  problem  of  the  chemistry  of  copper  sulphide 
enrichment  and  is  one  of  a series  of  studies  on  the  subject  of 
secondary  enrichment  which  are  being  pursued  by  this  laboratory 
in  cooperation  with  Professor  L.  C.  Graton  and  his  colleagues 
of  the  Harvard  Mining  School,  and  with  many  American  copper 
companies.2  In  this  investigation  the  authors  have  had  the  ad- 
vantage of  constant  advice  from  their  geological  colleagues  who 
are  at  present  actively  pursuing  the  investigation  of  the  copper 
sulphide  ores  from  the  geological  side. 

II.  GENERAL  PROCEDURE. 

Minerals  Studied. — It  became  evident  at  the  very  beginning  of 
our  experimental  work  that  the  complex  conditions  which  pre- 
vail in  the  enrichment  process  must,  for  laboratory  study,  be 
simplified  as  much  as  possible.  To  this  end,  the  reactions  of  each 
sulphide  have  been  separately  investigated.  There  is  reason  for 
believing  that  some  or  all  of  these  reactions  may  be  modified  in 
a mixture  of  sulphides,  and  some  of  these  mixtures  should  even- 
tually be  studied.  The  sulphides  chosen  for  our  work  were  covel- 
lite,  chalcocite,  pyrite,  pyrrhotite,  chalcopyrite,  bornite,  sphaler- 
ite, and  galena.  Enargite  also  is  of  sufficient  importance  to  be 
included,  but  on  account  of  its  chemical  complexity  we  deemed  it 
wise  to  omit  it  here. 

The  minerals  used  were  the  purest  obtainable.  They  were 
first  examined  microscopically  in  order  to  determine  if  impuri- 

la  This  brief  statement  may  require  revision  when  the  work  of  our  geolog- 
ical colleagues  is  published. 

2 See  Eng.  Min.  J .,  96,  885,  1913. 


4io 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


ties  were  present  and  also  to  identify  such  impurities.  Further, 
where  possible,  a purification  was  undertaken,  the  details  of 
which  are  given  in  the  proper  place  together  with  the  subsequent 
analyses. 

The  study  of  certain  synthetic  sulphides  which  have  the  advan- 
tage of  high  purity  was  included  along  with  the  natural  minerals ; 
but  we  may  anticipate  by  saying  that,  in  so  far  as  the  products 
formed  are  concerned,  no  difference  was  found. 

Sizing. — For  some  purposes  lumps  of  the  minerals  were  used 
so  as  to  note  the  color  of  the  product  formed,  and  also  to  note 
any  change  which  might  take  place  within  the  lump  itself ; for 
other  purposes  the  sulphides  were  ground  to  pass  through  a silk 
bolting  cloth  containing  ioo  meshes  to  the  linear  inch.  This  was 
done  to  present  a large  reacting  surface,  which  of  course  governs 
the  rate  of  alteration  of  the  mineral.3  For  comparison  work, 
the  sulphides  were  sized  and  only  that  portion  used  which  passed 
through  a silk  bolting  cloth  of  125  meshes  to  the  linear  inch  and 
was  caught  on  200  mesh.  Everything  finer  than  200  mesh  was 
rejected.  Closer  sizing  would  have  been  desirable,  but  the  small 
amount  of  pure  pyrrhotite  and  bornite  obtainable  necessitated 
this  wider  range.  After  the  sulphides  were  sized  in  the  manner 
indicated,  the  fine  flour  which  adheres  and  is  not  removed  by  bolt- 
ing was  removed  by  elutriation,  using  first  water  and  then  alco- 
hol, which,  owing  to  its  greater  viscosity,  will  hold  the  very  fine 
particles  in  suspension  longer  and  thus  permit  a more  effective 
separation  of  the  fine  flour  from  the  sized  material.  The  alcohol 
was  removed  by  washing  with  ether  and  the  latter  removed  by 
heating  at  40°.  The  sulphides  were  then  placed  over  sulphuric 
acid  in  a vacuum  desiccator  for  about  one  day,  after  which  they 
were  again  brought  on  the  200-mesh  bolting  cloth  and  those 
grains  removed  which  the  adhering  flour  had  previously  pre- 
vented from  going  through.  The  removal  of  this  fine  flour  is 
quite  important  if  comparable  surfaces  are  to  be  obtained. 

Solutions  Used. — Each  of  the  pure  sulphides  was  first  studied 
in  relation  to  its  behavior  with  copper  sulphate,  the  most  impor- 

3 For  influence  of  surface,  especially  in  reference  to  pyrrhotite,  see  F.  F. 
Grout,  Econ.  Geol.,  VIII.,  412. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  41 1 

tant  constituent  of  the  enriching  solution.  Then  the  influence  of 
each  of  what  we  believe  to  be  the  more  important  of  the  other 
constituents,  namely,  sulphuric  acid  and  ferrous  sulphate,  was 
investigated  as  far  as  at  present  feasible.  In  the  course  of  the 
work,  the  importance  of  cuprous  sulphate  became  evident  and  this 
was  studied  in  a preliminary  manner.  If  possible,  a more  de- 
tailed study  of  the  influence  of  both  cuprous  and  ferrous  sul- 
phates will  be  undertaken  at  some  later  time. 

Physical  Conditions.- — In  regard  to  the  physical  conditions  of 
the  experiments,  it  was  deemed  inadvisable  to  confine  ourselves 
to  natural  conditions.  We  have  kept  in  mind  the  general  chem- 
ical relations,  believing  it  important  that  the  chemistry  of  the 
sulphides  should  be  developed.  This  work  has  not,  therefore, 
been  carried  out  entirely  at  low  temperatures.  Indeed,  the  nature 
of  the  reactions  would  have  practically  precluded  such  a course. 
At  ordinary  temperatures  these  reactions  are  usually  very  slow 
and  the  reaction  products  within  a reasonable  time  small  in 
quantity.  Consequently,  the  results  obtained  at  ordinary  tem- 
peratures could  be  properly  interpreted  only  after  carrying  out  a 
series  of  experiments  at  higher  temperatures  ranging  from  ioo° 
to  200°  where  the  greater  speed  of  reaction  enabled  one  to  obtain 
adequate  quantities  of  the  reaction  products  within  a compara- 
tively short  time.  It  is  important  to  note  that,  as  a rule,  the 
chemical  reactions  in  these  systems  were  accelerated  rather  than 
changed  in  character. 

Examination  of  Product. — After  the  mineral  powders  had 
been  in  contact  with  the  solutions,  they  were  examined  micro- 
scopically and  analyzed.  The  powders  which  had  been  sized 
were  usually  examined  first  by  transmitted  light  to  see  if  any 
transparent  mineral  had  been  formed,  and  then  several  grains 
were  spread  out  on  an  object  slide  and  covered  with  melted  seal- 
ing wax  under  a cover  glass.  After  springing  off  the  cover  glass, 
the  grains  were  ground  down  and  polished.  Certain  fine  powders 
were  most  easily  examined  microscopically  after  being  com- 
pressed into  tablets.  When  a highly  polished  plunger  was  used, 


412 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


no  polishing  of  the  specimens  prepared  in  this  manner  was 
required.4 

A binocular  microscope  was  found  very  useful  in  making  color 
comparisons  and  in  determining  the  distribution  of  new  crystals. 
A comparison  eyepiece  also  was  used  in  making  color  com- 
parisons. 

Air  Excluded. — In  all  experiments  air  was  rigorously  ex- 
cluded; how  this  was  done,  together  with  the  apparatus  used, 
will  be  described  under  the  heading  which  follows. 

III.  APPARATUS. 

At  200°  the  experiments  were  carried  out  by  permitting  the 
substance  and  the  solution  to  react  within  sealed  Jena  glass  tubes 
which  were  heated  in  bombs  by  a resistance  furnace;  following 
very  much  the  same  procedure  as  that  described  in  a previous 
paper  from  this  laboratory.5  When  the  experiments  were  of 
short  duration  the  action  "on  the  glass  of  the  copper  sulphate  solu- 
tion and  that  of  the  sulphuric  acid  formed  during  the  experiment 
could  be  neglected ; many  of  the  experiments,  however,  extended 
over  a period  of  five  days  or  more.  Under  these  conditions  the 
Jena  glass  tubes  were  markedly  attacked.  To  overcome  this  diffi- 
culty, silica  tubes  of  the  smooth,  translucent  variety  were  used. 
These  tubes  had  an  internal  diameter  of  18  mm.  and  an  outside 
diameter  of  22  mm.  Tubes  of  this  size  were  sealed  before  a blast 
lamp  using  illuminating  gas  and  oxygen,  or  before  a torch  com- 
monly used  in  autogenous  welding.  Such  a torch,  however,  is 
best  suited  for  acetylene  gas  and  oxygen.  In  the  case  of  both  the 
Jena  glass  and  the  silica  tubes,  a tube  of  smaller  diameter  (about 
10  mm.)  was  sealed  on  to  the  larger  tube  and  then  constricted. 
This  constriction  was  about  25  mm.  long  with  an  internal  diam- 
eter of  about  1 mm.,  and  an  external  diameter  of  about  4 mm. 
The  constriction  is  necessary  in  order  to  properly  seal  the  reac- 
tion tube  after  it  has  been  evacuated.  The  tubes  were  carefully 

4 We  wish  to  express  our  thanks  to  Drs.  J.  Johnston  and  L.  H.  Adams  for 
their  aid  in  the  pressure  work. 

5 Allen,  Crenshaw,  Johnston,  and  Larsen,  Am.  J.  Sci.  (4),  33,  172,  1912. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


413 


evacuated  with  a May-Nelson  pump  and  at  the  same  time  were 
heated  with  water  at  about  70 °.  This  evacuation  must  be  done 
very  carefully  else  the  water  hammer  effect  which  always  makes 
itself  evident  becomes  so  violent  as  to  carry  some  of  the  solution 
and  at  times  some  of  the  sulphide  into  the  smaller  tube,  thus 
ruining  the  experiment  in  so  far  as  quantitative  work  is  con- 
cerned. When  the  constriction  has  the  internal  diameter  just 
given  and  a stopcock  is  placed  between  the  tube  and  the  pump, 
the  rate  of  evacuation  can  be  controlled  fairly  easily,  thus  over- 
coming the  difficulty  referred  to.  By  this  method  of  evacuation 
the  solution  in  the  tube  becomes  somewhat  more  concentrated  by 
evaporation,  but  the  amount  is  not  great  enough  to  seriously 
affect  the  results.  The  silica  tubes  were  sealed  off  at  the  con- 
striction by  placing  it  at  once  in  the  hot  flame  required;  when 
Jena  glass  tubes  were  used  the  constricted  portion  of  the  tube 
was  warmed  gently  before  sealing,  in  order  to  drive  away  from 
the  constriction  the  water  which  condensed  during  evacuation. 
Later  on  in  the  work  it  was  found  that  the  Jena  glass  could  be 
manipulated  quicker  and  better  when  using  a Baird  and  Tatlack 
“ oxyhydrogen  mixed  gas  jet”;  in  place  of  hydrogen  we  used 
illuminating  gas.  When  using  this  type  of  lamp  the  glass  must 
be  gently  heated  before  the  hot  flame  is  used  and  slowly  cooled 
and  “ smoked”  when  the  manipulations  are  finished. 

For  the  work  at  ioo°,  the  same  kinds  of  tubes  were  used  as  at 
200°,  but  it  was  found  desirable  to  have  a furnace  which  would 
permit  the  simultaneous  heating  of  a number  of  bombs  at  the 
same  temperature.  These  bombs  were  heated  by  circulating  oil 
at  the  proper  temperature  around  thin-walled  tubes  (boiler  tubes) 
into  which  bottoms  had  been  welded.  These  thin-walled  cylinders 
were  the  containers  for  the  bombs,  and  also  prevented  the  oil 
from  coming  in  contact  with  the  bombs.  The  oil6  used  for  this 
purpose  must,  of  course,  have  a high  “ flash  point.”  The  furnace 
in  which  the  heating  was  accomplished  is  illustrated  in  Fig.  24. 

6 “ Crisco  ” has  been  found  very  useful  in  this  laboratory,  at  and  above  50°. 
It  can  be  used  over  a long  period  of  time  at  temperatures  up  to  150°  and  inter- 
mittently up  to  200°.  At  200°  it  has  a tendency  to  “smoke”  and  also  to  attack 
the  steel  container  slightly. 


r 


4H  E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 

The  thoroughly  insulated  iron  cylinder  A,  40  in.  long,  was 
made  from  a piece  of  10  in.  pipe  into  which  a bottom  had  been 
welded.  The  eight  containers  for  the  bombs  were  fitted  into  the 


footed  base  B,  arranged  in  the  manner  indicated,  and  held  in 
place  at  the  top  by  means  of  the  perforated  ring  C.  A larger 
center  tube  D was  fitted  into  the  center  of  base  B and  ring  C and 
was  open  at  the  bottom.  This  tube  was  slotted  above  the  upper 
stirrer.  The  stirring  shaft  E,  carrying  the  two  stirrers  b and  c, 
was  mounted  in  the  bearings  d and  e and  was  prevented  from 
flipping  through  them  by  means  of  the  collar  /.  Several  steel 
washers  were  placed  between  the  collar  and  the  main  bearing  d. 
The  oil  (“Crisco”)  was  circulated  by  drawing  it  through  the 
bottom  of  the  center  tube  D,  and  discharging  into  the  outer 
cylinder  through  the  slots  in  upper  portion  of  D.  A supporting 
band  of  galvanized  iron  screen,  whose  mesh  was  J4  in.  square, 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4 1 5 

was  held  in  place  around  the  tubes  and  electrically  insulated  from 
them.  On  this  supporting  band  were  wound  in  parallel  helices 
six  “ Dubilier  ” resistance  tapes.  These  tapes  had  a resistance 
of  39  ohms  and  a carrying  capacity  of  3 amperes.  The  switch- 
board arrangement  was  such  that  the  various  resistance  tapes 
could  be  used  in  parallel  or  in  series,  and  any  one  of  them  could 
be  operated  through  a relay  which  in  turn  was  controlled  by  the 
pen  on  the  temperature  recording  instrument.  The  pen  was  elec- 
trically connected  in  such  a manner  that,  by  its  motion  to  and  fro 
across  the  recorder  paper,  the  relay  circuit  was  opened  and 
closed.  By  means  of  this  controlling  device,  the  temperature  of 
the  furnace  could  be  regulated  within  0.2 °. 

D is  a thoroughly  insulated,  removable  cover  carrying  the 
driving  pulley  and  the  upper  link  of  the  stirring  rod..  The  two 
parts  of  the  stirring  rod  were  connected  by  means  of  a V-shaped 
“ tongue-and-slot  ” coupling.  The  cover  was  heavy  enough  to 
prevent  wobbling  when  the  stirrer  was  in  motion. 

The  bombs  were  made  from  so-called  in.  double  extra 
heavy  pipe,  33  in.  long,  into  which  bottoms  were  welded.  These 
bombs  had  an  outside  diameter  of  1.90  in.  and  an  inside  diameter 
of  1. 125  in.  The  method  of  closure  was  essentially  the  same  as 
that  described  by  G.  W.  Morey7  and  is  shown  in  Fig.  24. 

Apparatus  Used  at  Ordinary  Temperatures. — At  ordinary 
temperatures,  the  enriching  solutions  react  with  the  sulphides 
very  slowly.  The  reactions  may,  however,  be  accelerated  by 
agitating  the  container  holding  the  sulphide  and  solution. 

The  shaking  machines  used  for  this  purpose  were  furnished  by 
the  International  Instrument  Company,  of  Cambridge,  Mass., 
and  proved  very  satisfactory  at  temperatures  up  to  50°.  The 
two  shakers  employed  were  placed  in  a well-insulated  box  pro- 
vided with  fans  for  circulating  the  air.  These  fans  were  attached 
to  the  spindle  carrying  the  shaker  arm.  The  heat  given  off  by 
the  motors  was  found  sufficient  to  maintain  a temperature  of 
40°±3°  in  the  thermostat.  At  50°,  the  shakers  were  driven 
by  a motor  placed  outside  of  the  thermostat  and  the  heat  sup- 

7 G.  W.  Morey,  Jr.  Am.  Chem.  Soc.,  XXXVI.,  217. 


f 


416  E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 

plied  by  electrical  resistance  units  which  were  controlled  by  a 
thermo-regulator  operating  through  a relay.  Under  these  con- 
ditions 50°  was  maintained  within  i°,  a regulation  closer  than 
actually  found  ncessary. 

The  container  used  is  shown  in  Fig.  25  and  consisted  of  a 
500  c.c.  “resistance”  glass  bottle,  which  was  steamed  out  before 


r 


Fig.  25. 

using.  The  glass  tube  “ B”  was  ground  into  the  neck  of  the 
bottle.  In  setting  up  an  experiment,  the  sulphide  and  solution 
were  placed  in  the  bottle,  the  tube  “ B”  made  wet  at  the  ground 
surface  and  inserted.  The  joint  at  “ d”  was  made  air  tight  with 
hard  “ Khotinsky  ” cement,  the  apparatus  was  then  evacuated  by 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4*7 

means  of  a May-Nelson  pump  while  the  bottle  was  heated  with 
water  at  40°.  The  stopcock  was  then  shut  and  the  apparatus 
sealed  off  at  the  constriction,  “c”.  About  18  mm.  of  the  con- 
stricted portion  were  left  on  “ B”  after  sealing  off.  At  the  end 
of  an  experiment,  the  constricted  portion  was  scratched  with  a 
file,  a rubber  tube  slipped  over  " B ” and  connected  with  a supply 
of  pure  C02.  The  constricted  portion  was  then  broken  off 
within  the  rubber  tube,  thus  permitting  the  filling  of  the  evacu- 
ated space  with  C02.  This  procedure  was  necessary  when  the 
solution  contained  ferrous  sulphate,  and  prevented  appreciable 
oxidation  during  the  interval  of  time  required  to  disconnect  at 
“ d,”  and  to  connect  the  bottle  to  the  apparatus  used  in  filtering 


O 

c= 


A 


Fig.  26. 


the  solution  in  an  atmosphere  of  C02.  In  some  of  the  experi- 
ments, ferrous  sulphate  and  cupric  sulphate  together  were  used  as 
initial  constituents  of  the  reacting  solution.  Under  these  con- 


418 


E.  G.  Z1ES,  E.  T.  ALLEN  AND  H.  E.  MERW.IN. 


ditions  it  was  found  necessary  to  use  the  apparatus  shown  in 
Fig.  26.  The  inner  tube  was  necessary  in  order  to  introduce  the 
ferrous  sulphate  solution  into  the  bottle  out  of  contact  with  air. 
The  ground  joint  at  “ d”  was  made  tight  with  “ Khotinsky ” 


cement.  At  the  end  of  the  experiment  the  tube  “ B”  was  with- 
drawn from  the  bottle  after  very  gently  warming  the  neck  in 
order  to  soften  the  cement. 

When  ferrous  sulphate  was  used  as  one  of  the  initial  constitu- 
ents of  the  reacting  solution,  it  was  prepared  by  passing  sulphur 
dioxide  through  a solution  of  ferrous  sulphate  in  order  to  reduce 
the  small  amount  of  ferric  iron  present,  the  S02  boiled  out,  and 
the  solution  cooled  in  an  atmosphere  of  pure  C02.  The  cooled 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4*9 


solution  was  then  transferred  to  bottle  A,  shown  in  Fig.  26,  by 
means  of  the  apparatus  shown  in  Fig.  27.  “ B”  is  a long-necked 
graduated  50  c.c.  flask  containing  the  solution  of  ferrous  sul- 
phate; S02  is  passed  in  for  a few  minutes,  the  flask  is  then  dis- 
connected from  the  S02  supply  and  the  contents  boiled  vigor- 
ously while  C02  is  passed  in.  After  complete  removal  of  the 
S02,  the  pinchcock  at  “b”  is  shut  and  contents  of  the  flask 
cooled  in  an  atmosphere  of  C02.  In  the  meantime,  the  sulphide 
and  a measured  volume  of  the  cupric  sulphate  solution  was  placed 
in  the  bottle  {( A”  which  was  then  evacuated.  The  C02  at  “a” 
was  then  disconnected  after  closing  pinchcock  “a”  and  glass 
tubes  at  “ a ” and  “ c ” filled  with  water  and  connected  by  means 
of  a rubber  tube.  On  opening  “a”  and  “ c”  the  contents  of 
“ B”  were  drawn  into  “A”  Water8  from  the  wash  bottle  " C” 
was  then  measured  into  “B”  and  drawn  into  “A”  until  the 
requisite  concentration  of  cupric  and  ferrous  sulphate  was  ob- 
tained. Toward  the  end  of  the  operation  it  was  at  times  neces- 
sary to  force  the  water  into  “A  ” under  C02  pressure.  This  was 
done  by  manipulating  the  proper  pinchcocks.  The  measuring  of 
the  solution  by  means  of  the  flask  “ B”  can,  of  course,  be  only 
approximate,  but  was  found  to  be  sufficiently  accurate  for  the 
purpose  in  hand.  When  the  necessary  amount  of  water  had  been 
added,  the  flask  “ A ” was  thoroughly  evacuated  while  the  con- 
tents were  heated  with  water  at  40°,  and  finally  sealed  off  at 
“ r and  n” 

With  this  general  idea  of  the  apparatus  used  and  the  plan 
followed  in  carrying  out  our  experiments,  let  us  proceed  to  the 
consideration  of  the  reactions  involved  when  the  enriching  solu- 
tions act  on  the  sulphides. 

IV.  REACTIONS  OF  THE  ENRICHING  SOLUTIONS  ON  THE 
INDIVIDUAL  SULPHIDES. 

I.  ACTION  OF  CUPRIC  SULPHATE  ON  COVELLITE  (CllS). 

In  the  course  of  our  experimental  work  it  was  found  that  this 
reaction  is  involved  whenever  the  sulphides  discussed  in  this 

8 Boiled  vigorously  to  remove  air  and  subsequently  cooled  in  pure  C02. 


420 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


paper  reacted  with  solutions  of  cupric  sulphate  under  the  condi- 
tions imposed  by  our  experiments,  consequently  it  was  thought 
desirable  to  begin  the  discussion  with  this  reaction. 

Stokes  heated  covellite  and  a io  per  cent,  solution  of  cupric 
sulphate  at  200-230°  C.9  The  reaction  tube  was  bent  to  form 
an  angle  of  120° ; one  arm  of  the  tube  was  heated  and  the  other 
cooled.  Under  these  conditions  Stokes  concluded  that  no  reac- 
tion took  place,  but  it  seems  that  he  came  to  this  conclusion 
because  no  metallic  copper  was  deposited  in  the  cold  arm  of  the 
tube;  whereas  when  pyrite  and  chalcocite  were  thus  treated, 
metallic  copper  had  formed.  In  order  to  obtain  additional  in- 
formation on  this  point  we  carried  out  a series  of  experiments 
at  200°,  using  first  a synthetic  covellite  and  later  a very  pure 
covellite  from  Butte,  Mont. 

A.  The  Reaction  at  200°. 

(a)  Experiments  on  Synthetic  Covellite  ( CuS ). 

This  covellite  was  prepared  by  passing  hydrogen  sulphide  into 
a hot  acidified  solution  of  copper  sulphate,  filtering,  washing  with 
acidified  H2S  water,  and  finally,  with  H2S  water  alone.  The 
precipitate  was  dried  and  then  heated  in  an  atmosphere  of  hydro- 
gen sulphide  at  3500.10  The  resulting  sulphide  was  crystalline, 
had  the  characteristic  indigo  blue  color  of  covellite,  and  analyzed 
as  follows : 


Found  by  analysis  Cu  = 66.43  per  cent.  S = 33.49  per  cent. 

Theoretical  for  CuS  Cu  = 66.47  Per  cent.  S = 33.53  per  cent. 


The  experiments  were  carried  out  in  silica  tubes  using  the 
synthetic  covellite  mentioned  above  and  a 5 per  cent,  solution  of 
CuS04.5H20.1:l  A preliminary  experiment  was  carried  out  in 
order  to  note  any  peculiar  difficulties  and  also  to  note  the  more 
apparent  physical  changes  which  take  place.  It  was  found  ex- 
tremely difficult  to  remove  the  solid  completely  from  the  tubes, 

9 H.  N.  Stokes,  Econ.  Geol.,  I.,  648,  1906. 

10  Posnjak,  Allen  and  Merwin,  Econ.  Geol.,  X.,  p.  527,  1915. 

11  See  footnote  12. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  42 1 

but  the  error  in  its  weight  was  easily  corrected,  after  the  analysis 
of  the  removable  portion,  by  dissolving  out  the  residue,  deter- 
mining the  copper  in  it,  and  calculating  the  equivalent  sulphide 
from  the  analysis  of  the  removable  portion. 

At  200°  it  was  found  that  our  synthetic  covellite  reacted  very 
readily  with  a solution  of  cupric  sulphate.  The  sulphide  at  the 
end  of  the  experiment  was  no  longer  indigo  blue,  but  had  be- 
come gray  like  chalcocite;  the  solution  had  become  colorless  and 
contained  sulphuric  acid.  Below  are  tabulated  the  initial  con- 
ditions of  the  experiments  and  the  analyses  of  the  resulting 
solutions. 

The  sulphuric  acid  noted  in  Table  I.  was  determined  by  titra- 
tion with  sodium  carbonate,  using  methyl  orange  as  an  indicator. 
If  the  solution  left  at  the  end  of  an  experiment  is  strongly  colored 

TABLE  I. 

Synthetic  Covellite  and  Cupric  Sulphate. 

Initial  Conditions:  Weight  of  covellite,  0.5000  g. ; solution,  20  c.c.  5 per  cent. 
CuS04-5H20  ;12  temperature,  200°. 


Analyses  of  Solutions. 

Exp. 

Duration,  Days. 

Copper,  g. 

Acid,  g. 

Initial  in  Solution. 

Final. 

Deposited. 

h2so4. 

! I 

2 

0.2560 

0.0860 

0.1700 

0.3488 

2 

2 

0.2560 

O.7788 

O.1772 

0.3590 

with  copper,  this  titration  can  not  be  made  accurately,  as  it  then 
becomes  difficult  to  determine  the  end  point.  After  carrying  out 
a preliminary  experiment,  we  found  that  the  final  concentration 
of  copper  could  be  reduced  by  the  reaction  between  covellite  and 
cupric  sulphate  to  a quantity  which  would  not  seriously  inter- 
fere with  the  determination  of  the  end  point.  All  copper  deter- 
minations were  carried  out  by  electrolysis.  The  powder,  after 

12  The  percentage  of  CuS04-5H20  given  in  these  and  in  subsequent  experi- 
ments is  only  approximate,  but  in  no  case  does  it  vary  sufficiently  to  vitiate 
the  conclusions  based  on  the  experiments.  The  accurately  determined  copper 
content  of  the  solution  is  always  noted. 


422 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


treatment  with  the  cupric  sulphate  solution,  was  examined  micro- 
scopically and  found  to  contain  a little  unaltered  covellite  and 
much  chalcocite,13  which  was  darker14  than  pure  chalcocite  and 
therefore  inferred  to  contain  dissolved  cupric  sulphide.  No 
metallic  copper  was  found. 

With  this  evidence  that  the  residues  contained  both  cupric  and 
cuprous  sulphide  and  no  metallic  copper,  we  felt  safe  in  calcu- 
lating the  amounts  of  cupric  and  cuprous  sulphide  from  the 
weight  of  the  residue  and  its  copper  content.  In  one  experiment, 
moreover,  both  copper  and  sulphur  were  determined  in  the  resi- 
due, and  their  summation  shows  that  no  other  substance  could 
have  been  present  in  sufficient  amount  to  seriously  change  the 
calculation  based  on  the  analysis. 


TABLE  II. 

Analyses  of  Residues  and  Calculations  Based  Thereon. 


Exp. 

Weight  of 
Residue, 
g- 

Per  Cent, 
of  Cu  in 
Residue. 

PerCent. 
of  S in 
Residue. 

Per  Cent, 
of  Cu2S.i5 

Per  Cent, 
of  CuS  by 
Difference. 

Weight  of 
Cu2S,  g. 

Weight  of 
CuS,  g. 

1 

2 

O.6402 

O.6461 

78.28 

78.38 

21-55 

88.20 

88.94 

II.80 

II.06 

0.5646 

O.5746 

O.0756 

O.0715 

The  Cu2S  was  calculated  on  the  basis  of  the  copper  present  in 
the  residue,  since  this  element  can  be  determined  more  accurately 
than  the  sulphur. 

Knowing  the  weight  of  the  initial  CuS  and  that  of  the  final 
CuS,  we  obtain  the  weight  of  the  covellite  which  reacted;  and, 
with  values  furnished  by  the  analysis  of  both  the  solution  and  the 
residue,  we  are  able  to  calculate  the  molecular  ratios. 

13  In  a number  of  our  experiments,  cuprous  sulphide  was  formed  above  930. 
While  it  is  true  that  cuprous  sulphide  above  930  is  in  the  isometric  form,  yet 
when  examined  at  ordinary  temperature  it  is  in  most  cases  in  the  ortho- 
rhombic form  of  ordinary  chalcocite.  Throughout  the  paper  “ chalcocite  ” is 
used  to  designate  cuprous  sulphide  in  accordance  with  , its  general  usage  in 
designating  natural  cuprous  sulphide. 

14Posnjak,  Allen  and  Merwin,  Econ.  Geol.,  X.,  506  (1915). 

15  Based  on  percentage  of  copper  in  residue. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  42  3 


TABLE  III. 

Weight  of  Covellite  which  Reacted  and  Molecular  Ratios. 


Weight  of  Covellite,  g. 

Molecular  Ratios  Based  on  Analyses  of  Solutions  and  Residues. 

Exp. 

IT  • ^ I 

Which 

H2SO4  Formed  . 

CuS  Which  Reacted 

CU2S  Formed 

Initial. 

r inai. 

Reacted. 

Cu  Deposited  ’ 

CU2S  Formed 

H2SO4  Formed" 

I 

0.5000 

0.0756 

O.4244 

1-33 

1.25 

1. 00 

2 

0.5000 

0.0715 

0.0715 

I.3I 

I.24 

1.02 

The  ratios  agree  well  with  those  demanded  by  the  following 
equation : 

5Q1S  -f-  3CUSO4  -j-  4H2O 4Cu2S  -f-  4H2SO4,  ( 1 ) 

in  which 

H2S04  CuS  Cu2S 

Cu  ~ 1-33 : +:  Cu2S  ~ 1251  HsSOi  “ T' 

% 

Inasmuch  as  the  tubes  were  evacuated,  the  results  show  that 
the  sulphur  in  the  covellite  is  oxidized  at  the  expense  of  the 
oxygen  in  the  cupric  sulphate. 

(&)  Experiments  on  Natural  Covellite  and  Cupric  Sulphate 

at  200° C . 

Experiments  were  likewise  carried  out  on  a very  pure  covel- 
lite from  Butte,  Mont.,  which  analyzed  as  follows : 


Sample  Cu  = 66.43  per  cent.  S = 33.35  per  cent. 

Theoretical  for  CuS  Cu  = 66.47  Per  cent.  S = 33.53  per  cent. 


Microscopic  examination  of  the  polished  sulphide  revealed  the 
presence  of  very  minute  quantities  of  chalcopyrite  and  quartz. 
These  experiments  were  carried  out  as  follows : The  covellite, 
ground  to  pass  through  a 100-mesh  bolting  cloth,  was  permitted 
to  react  in  silica  tubes  with  such  an  amount  of  a 2 per  cent, 
cupric  sulphate  solution  as  would  convert  only  a portion  of  the 
covellite  into  cuprous  sulphide.  This  process  was  repeated  until 
the  analysis  showed  that  the  residue  was  practically  pure  cuprous 
sulphide.  Experiments  3,  4,  and  5 in  Table  IV.  represent  the 
first  stage  of  the  reaction;  6 and  7 the  second  stage.  The  latter 
were  carried  out  by  mixing  the  reground  residue  from  3,  4,  and 
5 and  again  treating  with  a 2 per  cent.  CuS04  solution ; 8 repre- 


424 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


sents  the  results  obtained  on  treating  the  reground  residues  from 
6 and  7 in  the  same  manner;  9 represents  the  final  step.  The 
analyses  were  made  and  the  ratios  determined  in  the  same  manner 
as  indicated  under  the  experiments  with  synthetic  covellite. 

TABLE  IV. 

Natural  Covellite  and  Cupric  Sulphate. 

Initial  Conditions:  Covellite  from  Butte,  Mont.;  solution,  30  c.c.  2 per  cent. 

CuS04-5H20;  temperature,  200°. 


Exp. 

Duration, 

Days. 

Weight  of 
Covellite, 

S • 

Analyses  of  Solutions. 

Copper,  g. 

Acid,  g. 
H2SO4. 

Mol.  Ratio : 

h2so4 

Initial  in 
Solution. 

Final. 

Deposited. 

Cu  Deposited' 

3 

2 

2.0000 

0.1536 

0.0025 

0.I5II 

0.3060 

I.3I 

4 

2 

0.0020 

O.1516 

0.3073 

I.3I 

5 

2 

0.0015 

O.I52I 

O.3083 

I-3I 

6 

2 

1.5000 

0.0012 

O.1524 

0.3105 

1.32 

7 

2 

4 4 

0.0018 

O.1518 

O.3094 

1.32 

8 

2 

0.8000 

O.O418 

0.III8 

O.2278 

1.32 

9 

I 

0.6000 

0.0862 

0.0674 

0.1374 

1.32 

The  residue  obtained  at  the  end  of  each  experiment  was  ex- 
amined microscopically  as  before;  no  metallic  copper  was  found 
in  any  of  the  residues.  In  experiment  9,  the  microscopic  evi- 
dence was  clear  that  the  original  covellite  had  altered  completely 
into  chalcocite ; in  the  other  experiments  some  covellite  remained 
unattacked.  All  of  the  residues  were  now  analyzed  for  copper, 
and  in  the  first  three  experiments  the  amounts  of  cupric  and 
cuprous  sulphide  were  calculated  in  the  same  manner  as  de- 
scribed on  page  423. 

TABLE  V. 

Analyses  of  Residues. 


Exp. 

Weight  of 
Residue,  g. 

Per  Cent,  of 
Cu  in  Residue. 

Per  Cent,  of 
Cu2S  Based  on 
Cu  in  Residue. 

Per  Cent,  of 
CuS  by 
Difference. 

Weight  of 
Cu2S,  g. 

Weight  of 
CuS,  g. 

3 

2.1256 

69.47  ‘ 

22.41 

77-59 

O.4761 

1.6495 

4 

2.1260 

69.62 

23-53 

76.47 

0.5000 

I.6256 

5 

2.1265 

69.65 

23.76 

76.25 

O.5051 

I.6214 

6 

73-65 

7 

73.58 

8 

78.52 

9 

79-75 

SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


425 


The  theoretical  percentage  of  copper  in  cuprous  sulphide  is 
79.86  per  cent. ; the  analysis  of  the  residue  in  experiment  9 there- 
fore confirms  the  observation  made  with  the  microscope  that 
alteration  was  complete. 

In  the  first  three  experiments,  knowing  the  weight  of  the 
initial  CuS  and  that  of  the  final  CuS,  we  obtain  the  weight  of 
the  covellite  which  reacted;  and  with  values  furnished  by  the 
analysis  of  both  the  solution  and  the  residue,  we  are  able  to  cal- 
culate the  molecular  ratios. 


TABLE  VI. 

Weight  of  Covellite  which  Reacted  and  Molecular  Ratios. 


Exp. 

Weight  of  Covellite 

g. 

Molecular  Ratios  Based  on  Analyses  of  Solu- 
tions and  Residues. 

Initial. 

Final. 

Which  Re- 
acted. 

H2SO4  Formed 

CuS  which 
Reacted 

CU2S  Formed 

Cu  Deposited 

CU2S  Formed 

H2SO4  Formed 

3 

2.0000 

I.6495 

0.3505 

I.3I 

1.25 

O.96 

4 

2.0000 

I.6256 

0-3744 

I.3I 

I.25 

1. 00 

5 

2.0000 

I.6214 

O.3786 

I.3I 

1.25 

1. 01 

Discussion. — All  of  the  evidence,  both  microscopic  and  chem- 
ical, points  clearly  to  the  following  reaction  as  representing  the 
alteration  of  covellite  into  cuprous  sulphide  (Cu2S)  in  the  pres- 
ence of  cupric  sulphate : 

5G1S  -f  3CUSO4  + 4H20  = 4Cu2S  + 4H2S04.  ( 1 ) 

The  evidence  also  brings  out  the  fact  that  the  reaction  repre- 
sented by  this  equation  holds  both  for  the  synthetic  and  natural 
covellite.  Inasmuch  as  the  complete  alteration  of  our  natural 
covellite  into  cuprous  sulphide  was  carried  out  in  several  stages 
and  inasmuch  as  the  molecular  ratio  H2S04/ (Cu  deposited)  is  the 
same  at  the  end  of  each  stage,  the  above  equation  represents  the 
reaction  which  takes  place  irrespective  of  the  amount  of  covellite 
still  present.  In  this  respect  the  reaction  between  covellite  and 
cupric  sulphate  differs  markedly  from  the  reactions  between  the 
iron  and  iron-copper  sulphides  and  cupric  sulphate  which  are 
described  later  in  this  paper.  There  it  will  be  shown  that  chang- 


426 


E.  G.  Z1ES,  E.  T.  ALLEN  AND  H.  E M ERWIN. 


ing  the  amount  of  the  reacting  sulphide  also  changes  the  nature 
of  the  reaction. 

Especial  attention  should  be  called  to  the  fact  that  equation 
(i)  holds  so  long  as  covellite  is  still  present  in  the  residue  to- 
gether with  cuprous  suphide,  as  was  the  case  in  experiments  3 to 
8.  It  also  holds  when  only  cuprous  sulphide  is  present,  provided 
the  conditions  of  the  experiment  are  those  in  9,  Table  IV.,  where 
the  experiment  was  of  short  duration  and  the  final  concentration 
of  the  cupric  sulphate  solution  small.  Subsequently  it  will  be 
shown  that  cuprous  sulphide  itself  is  attacked  by  cupric  sulphate, 
the  oxidation  of  the  sulphur  in  the  cuprous  sulphide  resulting  in 
metallic  copper  and  sulphuric  acid. 

B.  Covellite  and  Cupric  Sulphate  at  ioo°  C. 

Covellite  and  a solution  of  cupric  sulphate  will  also  react  at 
ioo°  though,  naturally,  more  slowly  than  at  200 0 ; but  the 
amount  of  the  reaction  products  is  sufficiently  large  to  permit 
our  making  the  assertion  that  the  nature  of  the  reaction  is  the 
same ; namely,  the  sulphur  or  part  of  the  sulphur  in  the  covellite 
is  oxidized,  yielding  cuprous  sulphide  and  sulphuric  acid. 

TABLE  VII. 

Covellite  and  Cupric  Sulphate. 

Initial  Conditions:  1.0000  g.  covellite  from  Butte,  Mont.  (100  mesh  and  finer)  ; 
solution,  25  c.c.  5 per  cent.  CuS04-5H20;  temperature,  ioo°. 


Analysis  of  Solutions. 

Exp. 

Duration, 

Days. 

Copper,  g. 

Molecular 

Initial  in 
Solution. ' 

Final. 

Deposited. 

Acid,  g. 

Ratio  W 
Cu 

10 

24 

0.3179 

O.2638 

O.OS41 

O.IIOO 

I.32 

The  residue  obtained  in  this  experiment  was  compressed, 
polished,  and  examined  mineralographically.16  The  pressure  of 
chalcocite,  together  with  a large  amount  of  unaltered  covellite, 
was  noted.  The  cuprous  sulphide  thus  noted  and  the  molecular 

16 Joseph  Murdoch,  “Microscopical  Determination  of  the  Opaque  Minerals,” 
page  3.  (John  Wiley  & Sons,  New  York.) 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


427 


ratio  shown  above  prove  that  the  same  equation  as  before  repre- 
sents the  reaction  between  covellite  and  cupric  sulphate,  namely : 

5Q1S  -f-  3Q1SO4  -f  4H20  — 4Cu2S  + 4H2S04.  ( 1 ) 

C.  Action  of  Cupric  Sulphate  on  Covellite  at  25-40°. 

We  now  desired  to  learn  if  a 5 per  cent,  solution  of  cupric 
sulphate  will  react  with  covellite  at  temperatures  ranging  from 
25-40°.  In  an  experiment  extending  over  a period  of  two 
months,  so  little  copper  was  lost  by  the  solution  of  cupric  sul- 
phate that  we  must  limit  ourselves  to  the  statement  that  covellite 
and  cupric  sulphate  probably  do  react  at  ordinary  temperatures, 
though  very  slowly.  In  our  experiments  with  sphalerite  and 
galena,  detailed  later  in  this  paper,  we  were  able  to  show  that  the 
covellite  which  replaced  these  minerals  reacted  much  faster  at 
ordinary  temperatures  with  a solution  of  cupric  sulphate  than 
when  grains  of  natural  covellite  are  used,  and  that  the  reaction  is 
represented  by  equation  ( 1 ) ; the  increase  in  reactivity  is  pre- 
sumably due  to  the  finer  state  of  division  of  the  cupric  sulphide.17 

2.  COVELLITE  AND  CUPROUS  SULPHATE. 

Covellite  can  alter  into  chalcocite  in  a manner  other  than  by 
oxidation,  as  shown  by  the  following  qualitative  experiments : 

Covellite  ground  to  pass  through  a 200-mesh  bolting  cloth  was 
mixed  with  metallic  copper  in  the  form  of  short  pieces  of  wire 
and  allowed  to  react  with  a 5 per  cent,  solution  of  cupric  sulphate 
at  ioo°.  At  the  end  of  three  days,  a part  of  the  residue  was 
imbedded  in  sealing  wax,  polished,  and  examined.  The  covellite 
had  altered  completely  into  chalcocite  with  which  a little  well- 
crystallized  cuprite  and  unattacked  metallic  copper  were  ad- 
mixed.18 The  cuprous  copper19  which  is  present  under  the  condi- 

17  The  difference  is  possibly  due  also  tp  the  electrolytic  action  caused  by  a 
difference  in  potential  between  galena  or  sphalerite  and  the  covellite,  thus 
resembling  the  effect  found  by  Gottschalk  and  Buehler  in  their  work  on  the 
oxidation  of  the  sulphides  (Econ.  Geol.,  VII.,  15,  1912). 

18  The  presence  of  this  cuprite  is  accounted  for  on  page  446. 

19  Foerster  and  Seidel,  Z.  anorg.  Chem.,  14,  106,  1897 ; Richards,  Collins  and 
Heimrod,  Z.  phys.  Chem.,  32 , 321,  1900;  Abel  Emil,  Z.  anorg.  Chem.,  26,  381, 
1901. 


428 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


tions  of  the  experiment  apparently  played  an  important  part  in 
this  reaction,  but  in  view  of  the  fact  that  the  copper  and  covellite 
were  in  contact  with  one  another,  we  suspected  that  some  electro- 
lytic action  had  taken  place.  In  order  to  obtain  further  evidence 
as  to  the  nature  of  the  reaction,  the  following  experiments  were 
carried  out: 

TABLE  VIII. 

Influence  of  Cuprous  Sulphate. 

Initial  Conditions:  Weight  of  CuS,  1.0000  g. ; solution,  25  c.c.  5 per  cent. 
CuS04-5H20. ; temperature,  ioo°. 


Analyses  of  Solutions. 

Exp. 

Duration, 

Days. 

Covellite,  Butte,  Mont. 

200  Mesh  and  Finer. 

Copper,  g. 

Initial  in 
Solution. 

Final. 

Lost  by 
Solution. 

Acid,  g. 

II 

3 

With  metallic  Cu 

0.3179 

0.3179 

O.3190 

O.2638 

None 

Trace 

IO 

24 

No  metallic  Cu 

0.0541 

O.IIOO 

The  metallic  copper  used  in  experiment  1 1 was  in  the  form  of 
short  pieces  of  wire  and  was  suspended  in  the  solution  out  of 
contact  with  the  covellite;  in  experiment  10,  no  metallic  copper 
was  used.  The  residue  in  experiment  11  was  examined  micro- 
scopically and  it  was  shown  that  the  covellite  had  completely 
altered  to  chalcocite,20  whereas  in  experiment  10,  which  extended 
over  a period  of  24  days,  it  was  estimated  that  75  per  cent,  of  the 
covellite  was  still  present,  showing  a great  difference  in  the  ease 
with  which  covellite  is  altered  in  the  two  instances.  Still  more 
striking,  however,  is  the  fact  that  the  solution  in  11  lost  none 
of  its  copper;  as  a matter  of  fact  it  actually  gained  a little;21 
also,  only  a trace  of  acid  was  present,  whereas  in  10  the  solution 
lost  a considerable  amount  of  copper  and  contained  a considerable 
amount  of  sulphuric  acid.  It  is  well  known  that  a solution  of 
cupric  sulphate  will  dissolve  metallic  copper  and  that  such  a solu- 
tion contains  some  copper  in  the  cuprous  condition ; inasmuch  as 
in  experiment  11  metallic  copper  was  present  and  none  was 

20  Unattacked  metallic  copper  was  also  present. 

21  Due  to  the  solubility  of  copper  in  copper  sulphate ; see  references  quoted, 
page  428. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  429 


present  in  io,  it  is  evident  that  the  cuprous  copper  was  responsible 
for  the  difference  in  the  rate  of  the  alteration  into  cuprous  sul- 
phide. Keeping  in  mind  the  fact  that  no  acid  was  formed  in 
ii  we  believe  that  we  are  justified  in  writing  the  following 
equation : 

CuS  -j-  Cu2S04  = Cu2S  + CuS04.  (2 ) 

It  seems  very  probable  from  the  foregoing  that  cuprous  sulphide 
under  the  conditions  of  the  experiment  is  more  insoluble  than 
cupric  sulphide.  The  reaction  in  the  case  of  experiment  10  has 
been  discussed  on  page  427. 

The  alteration  of  covellite  to  cuprous  sulphide  in  the  presence 
of  cuprous  copper  takes  place  readily  also  at  ordinary  tempera- 
tures, as  shown  by  a qualitative  experiment  carried  out  at  25 0 C., 
the  initial  conditions  of  which  were  the  same  as  those  in  experi- 
ment 1 1 on  page  428.  The  experiment  extended  over  a period  of 
six  days;  the  residue  was  examined  microscopically  and  it  was 
found  that  extensive  alteration  of  the  covellite  into  cuprous  sul- 
phide had  taken  place. 

In  all  probability,  cuprous  copper  plays  an  important  role  in 
the  alteration  of  covellite  to  cuprous  sulphide.  Thus  in  the  reac- 
tion between  covellite  and  cupric  sulphate,  the  oxidation  of  the 
sulphur  of  covellite  to  sulphuric  acid  must  be  accompanied  by 
reduction  of  cupric  sulphate  to  cuprous  sulphate,  which  we  have 
just  seen  reacts  very  readily  indeed  with  covellite.  It  appears 
probable  then,  that  the  reaction  between  covellite  and  cupric  sul- 
phate is  only  summarized  by  equation  ( 1 ) and  in  reality  proceeds 
in  two  stages,  namely, 

CuS  + 7CuS04  + 4H20  = 4Cu2S04  + 4H2S04.  (3) 

CuS  + Cu2S04  = Cu2S  + CuS04.  (2) 

The  experiments  show  that  the  reaction  represented  by  (2)  is 
more  rapid  than  that  represented  by  (3).  The  subject  of  the 
influence  of  cuprous  copper  is  worthy  of  a more  extended  investi- 
gation, especially  along  physico-chemical  lines. 


430 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


3.  ACTION  OF  CUPRIC  SULPHATE  ON  CHALCOCITE,  Cu2S. 

It  was  found  in  this  laboratory  by  Dr.  J.  L.  Crenshaw22  that 
chalcocite  is  attacked  at  300 0 by  a 10  per  cent,  cupric  sulphate 
solution,  yielding  metallic  copper.  In  addition  to  metallic  copper, 
basic  copper  sulphate,  resulting  from  the  hydrolysis  of  the  cupric 
sulphate,  is  also  formed.  We  repeated  the  experiment  and  con- 
firmed the  results.  The  same  experiment  was  also  carried  out  at 
200°  on  a very  pure  chalcocite  which  analyzed  as  follows : 

Chalcocite23  from  Butte,  Mont. . Cu  = 79.67,  S = 20.16,  Fe  = o.i4,  Si02  = 0.09 
Calculated  for  Cu2S  79-86,  20.14 

At  the  end  of  ten  days  the  residue  was  examined  microscopically 
and  the  presence  of  well-crystallized  metallic  copper  and  basic 
copper  sulphate  noted.  No  cupric  sulphide  was  found.  Pos- 
sibly cuprite  was  also  present  but  we  have  not  as  yet  been  able 
to  prove  this  point.  Sulphuric  acid  was  found  in  the  solution.  It 
is  impossible  at  present  to  analyze  this  complex  mixture,  and 
additional  work  must  be  carried  out  in  order  to  simplify  the 
reaction  before  quantitative  work  can  be  undertaken.  The  im- 
portant feaures  brought  out  by  the  qualitative  work  are  the  for- 
mation of  metallic  copper  and  the  fact  that  no  covellite  was 
formed  by  the  reaction. 

Stokes24  tried  the  action  of  cupric  sulphate  on  chalcocite  at 
225 0 with  somewhat  different  results.  His  apparatus  was  a hot- 
cold  system,  and,  according  to  him,  copper  was  deposited  at  the 
cold  end  of  the  tube  and  covellite  at  the  hot  end.  The  difference 
in  apparatus  should  make  no  difference  in  the  results  except  that 
under  Stokes’s  conditions  the  reaction  might  be  expected  to  be  more 
rapid.  In  view  of  the  fact  that  in  our  experimental  work  it  has 
been  shown  that  covellite  reacts  with  cupric  sulphate  under  simi- 
lar conditions  to  form  cuprous  sulphide,  and  since  we  have  shown 
that  covellite  reacts  very  readily  indeed  with  cuprous  sulphate25 

22  Unpublished  work. 

23  Analyzed  by  Dr.  E.  Posnjak;  see  Posnjak,  Allen  and  Merwin,  Econ. 
Geol.,  Vol.  X.,  1915,  p.  508,  analysis  No.  3. 

24  Stokes,  H.  N.,  Econ.  Geol.,  Vol.  I.,  1906,  p.  648. 

25  See  page  428. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


431 


to  form  cuprous  sulphide,  it  is  difficult  to  understand  how  covel- 
lite  could  be  formed  in  the  reaction  between  chalcocite  and  cupric 
sulphate. 

4.  ACTION  OF  CUPRIC  SULPHATE  ON  PYRITE,  FeS2. 

As  pyrite  is  one  of  the  commonest  sulphide  minerals  in  de- 
posits that  have  undergone  secondary  copper  enrichment,  the 
necessity  for  the  study  of  its  reaction  with  cupric  sulphate  solu- 
tions is  obvious. 

In  the  excellent  work  of  Stokes26  on  this  reaction,  it  was 
shown  that  cupric  or  cuprous  sulphides  or  both  were  formed,  and 
he  believed  that  the  cuprous  ion  played  an  important  part  in  the 
reaction.  The  end  products  obtained  in  the  reaction  just  men- 
tioned were  of  such  a complex  nature,  however,  as  to  preclude 
any  accurate  work  on  the  problem.  We  repeated  the  experi- 
ments at  200°  under  the  same  conditions  and  obtained  the  same 
products,  namely,  unoxidized  pyrite,  sulphides  of  copper,  cuprite, 
metallic  copper,  and  hematite,  in  the  residue;  ferrous  sulphate 
and  sulphuric  acid  in  the  solution.  Our  efforts  were  now  directed 
towards  simplifying  this  complicated  state  of  affairs. 

A.  The  Reaction  at  200° . 

( a ) The  Alteration  of  Pyrite  to  Chalcocite. 

This  was  finally  accomplished  by  permitting  the  cupric  sul- 
phate to  react  with  pyrite  mixed  with  three  times  its  volume  of 
pure  quartz.  The  quartz  used  in  these  experiments  was  carefully 
freed  from  iron  by  repeatedly  digesting  it  with  hot  hydrochloric 
acid;  the  acid  was  removed  by  washing  with  hot  water.  The 
quartz  was  sized  between  60  and  80  mesh ; in  no  experiment  was 
the  pyrite  larger  than  100  mesh,  therefore  the  quartz  could  be 
separated  from  the  pyrite  and  its  alteration  products  by  simply 
sifting  the  quartz  through  a 100-mesh  bolting  cloth.  The  quartz 
was  used  in  order  to  separate  the  grains  of  the  finely  divided 
pyrite  thus  permitting  the  cupric  sulphate  to  circulate  more  freely 

20  Stokes,  H.  N.,  Bull.  U.  S.  Geol.  Survey,  No.  186,  p.  41 ; also  Econ.  Geol., 
Vol.  I.,  p.  644. 


432 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


around  the  grains  and  also  exposing  a greater  surface  of  the 
pyrite  to  the  attacking  solution.  When  the  experiments  were 
carried  out  under  these  conditions,  neither  hematite,  cuprite,  nor 
metallic  copper  was  found  at  the  end  of  the  experiment.  The 
pyrite  is  covered  with  a coating  of  one  or  both  of  the  sulphides 
of  copper.  This  coating  adheres  firmly  to  the  pyrite,  resembling 
in  this  respect  pyrite  partially  replaced  in  nature  by  covellite  or 
chalcocite.  The  similarity  was  again  shown  by  the  fact  that  no 
measurable  crystals  could  be  observed  although  the  appearance  of 
the  residue  under  the  microscope  led  one  to  believe  that  the  de- 
posit was  crystalline. 

(i)  Methods  of  Analysis. 

Analysis  of  the  Enrichment  Products  Replacing  the  Pyrite. — 
The  residues  obtained  in  these  experiments  were  compressed  into 
tablets,  using  a pressure  of  12,000  atmospheres;  the  plunger  of 
the  hydraulic  press  was  highly  polished,  consequently,  the  com- 
pressed tablet  had  a highly  polished  surface,  thus  obviating  the 
polishing  required  previous  to  the  mineralographic  examination. 
This  procedure  was  found  necessary  because  the  pyrite  in  the 
compressed  residue  invariably  tore  away  from  the  softer  copper 
sulphides  as  soon  as  any  attempt  was  made  to  polish  the  com- 
pressed tablet  in  the  usual  manner.  This  microscopic  examina- 
tion proved  very  useful  indeed  when  the  coating  on  the  pyrite  was 
of  measurable  thickness,  but  in  many  experiments  the  coatings 
were  so  thin  as  to  give  iridescent  effects.  Under  these  conditions 
the  evidence  furnished  by  the  microscope  could  not  be  relied 
upon.  It  consequently  became  necessary  to  find  some  chemical 
method  for  identifying  the  alteration  products  on  the  pyrite,  and 
also  for  checking  up  the  evidence  furnished  by  the  microscope 
under  the  more  favorable  conditions.  We  succeeded  in  finding 
such  a method  and  shall  describe  it  in  detail  below,  since  by  means 
of  it  we  were  able  to  positively  identify  the  enrichment  products 
which  replaced  pyrite,  chalcopyrite,  sphalerite,  and  galena.27  It 

27  The  alteration  products  formed  on  bornite  could  not  be  determined  be- 
cause bornite  itself  is  easily  attacked  by  the  reagent  used. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  433 


thus  proved  a very  potent  factor  in  furthering  the  entire  investi- 
gation. 

Chemical  Method  for  Identifying  the  Alteration  Products. — It 
is  known  that  covellite  and  chalcocite  are  soluble  in  a 30  per 
cent,  solution  of  potassium  cyanide  and  the  statement  is  made  in 
the  literature  that  pyrite  is  not  attacked  by  a solution  of  this 
strength.  We  investigated  this  subject  rather  fully  and  found 
that  pyrite  is  attacked  by  a 30  per  cent,  solution  of  potassium 
cyanide,  especially  so  when  it  is  as  finely  divided,  as  is  the  case  in 
our  experiments.  We  found,  however,  that  a 5 per  cent,  solution 
will  dissolve  the  sulphides  of  copper  formed  on  pyrite  and  if  the 
analysis  is  carried  out  under  the  following  conditions,  the  pyrite 
is  attacked  to  such  a slight  extent  that  the  error  thus  introduced 
can  be  safely  neglected.  The  pyrite  and  its  alteration  products 
are  treated  in  the  cold  with  a 5 per  cent,  solution  of  potassium 
cyanide  and  the  solution  filtered  as  soon  as  the  coating  on  the 
pyrite  is  dissolved.  The  pyrite  is  then  washed  once  with  a 1 per 
cent,  solution  of  potassium  cyanide,  no  effort  being  made  to 
wash  thoroughly.  The  filtering  and  washing  should  be  done  as 
quickly  as  possible  in  order  not  to  expose  the  pyrite  to  the  action 
of  the  solution  any  longer  than  necessary.  The  volume  of  the 
filtrate  should  be  kept  within  50  c.c.  in  order  to  avoid  the  use  of 
excessively  large  amounts  of  fuming  nitric  acid,  which  is  used  in 
the  oxidation  of  the  sulphur  in  the  filtrate.  The  procedure  here 
described  was  slightly  modified  when  the  alteration  products  on 
chalcopyrite,  sphalerite,  and  galena  were  removed  and  the  reader 
is  referred  to  the  description  of  the  experimental  work  on  those 
sulphides  for  such  modifications.  The  solution  of  the  sulphides 
of  copper  in  potassium  cyanide  whether  derived  from  the  separa- 
tion of  the  alteration  products  from  pyrite,  chalcopyrite,  sphaler- 
ite, or  galena  was  analyzed  in  the  following  manner : 

The  filtrate  is  transferred  to  a 500  c.c.  flask  to  which  is  at- 
tached a reflux  condenser  which  is  fitted  to  the  flask  by  means  of 
a ground  joint.  Fuming  nitric  acid  is  then  poured  into  the  flask 
through  the  condenser  in  about  5 c.c.  portions,  until  50  c.c.  are 
added,  cooling  the  flask  throughout  this  procedure.  If  the  entire 


434 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


amount  of  acid  is  added  at  one  time,  the  contents  of  the  flask 
become  so  hot  as  to  partly  clot  the  sulphur  that  separates.  This 
condition  must  be  carefully  guarded  against,  as  it  unduly  pro- 
longs the  oxidation.  If  the  operation  is  carried  out  as  indicated, 
the  sulphur  remains  finely  divided.  The  contents  of  the  flask  are 
next  heated  at  about  90 °,  taking  great  care  not  to  permit  the 
temperature  to  rise  high  enough  to  melt  or  even  clot  the  sulphur. 
After  four  hours’  heating,  the  sulphur  is,  as  a rule,  completely 
oxidized,  but  to  make  sure,  the  contents  of  the  flask  were  usually 
heated  over  night.  If  a small  amount  of  sulphur  should  persist, 
its  oxidation  can  be  hastened  by  adding  a small  portion  of  hydro- 
chloric acid  free  from  sulphur  and  again  heating.  After  the  oxi- 
dation is  completed  the  contents  of  the  flask  are  transferred  to  an 
evaporating  dish  and  evaporated  to  dryness  and  then  evaporated 
twice  with  hydrochloric  acid  to  decompose  all  nitrates.  The  pre- 
cipitation of  the  sulphur  as  barium  sulphate  after  removal  of 
the  copper  as  oxide28  by  double  precipitation  from  boiling  solu- 
tion with  sodium  carbonate  was  carried  out  according  to  the 
method  developed  by  Allen  and  Johnston.29  When  the  amount  of 
sulphur  was  small — say  from  5 to  25  milligrams — then  the  barium 
sulphate  was  precipitated  in  a volume  of  solution  of  150  c.c., 
which  was  made  distinctly  acid,  care  being  taken  to  avoid  a large 
excess;  the  requisite  amount  of  barium  chloride  was  added  at 
once  to  the  hot  solution.  No  correction  for  solubility  was  made 
under  these  conditions.  The  copper,  both  that  precipitated  by  the 
sodium  carbonate  and  that  found  in  the  filtrate  from  the  barium 
sulphate,  was  determined.  The  copper  in  the  filtrate  from  barium 
sulphate  was  precipitated  with  hydrogen  sulphide  under  the  proper 
conditions ; dissolved  in  nitric  acid  and  added  to  that  obtained  by 
dissolving  the  precipitated  oxide  of  copper  in  nitric  acid,  after 
which  the  total  copper  was  determined  by  electrolysis.  If  the  pre- 
cipitation of  the  barium  sulphate  has  been  carefully  done  it 
should  contain  no  copper. 

28  Phenolphthalein  was  used  to  indicate  when  sufficient  sodium  carbonate 
had  been  added ; a large  excess  of  the  precipitant  is  to  be  avoided.  Subse- 
quently, when  the  filtrates  are  acidified  previous  to  precipitation  with  barium 
chloride,  methyl  orange  was  used  as  the  indicator. 

29  Allen  and  Johnston,  Jour.  Ind.  Eng.  Chem.,  Vol.  2,  No.  5,  1910,  p.  196. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  43  5 


(2)  Results  at  200°. 

Having  simplified  the  reaction  so  as  to  exclude  the  interfering 
reaction  products  mentioned  on  page  431  and  having  found  a 
method  for  analyzing  the  alteration  products  on  the  pyrite,  we 
proceeded  to  carry  out  the  following  experiment:  Pyrite  from 
Leadville,  mixed  with  three  times  its  volume  of  pure  quartz,  was 
exposed  in  a silica  tube  for  a period  of  nine  days  to  the  action  of 
a 5 per  cent,  solution  of  cupric  sulphate.  The  pyrite  used  in  this 
experiment  was  examined  microscopically  and  shown  to  be  very 
pure;  a chemical  analysis  revealed  the  presence  of  0.1  per  cent, 
of  Cu  and  0.2  per  cent,  of  some  residue  insoluble  in  nitric  acid. 
The  amounts  of  the  impurities  found  by  the  chemical  analysis  are 
far  too  small  to  be  of  any  consequence  in  altering  the  deductions 
made  on  the  basis  of  the  results  obtained  in  any  experiment  in 
which  this  pyrite  was  used. 

The  residue  obtained  in  this  experiment  was  examined  micro- 
scopically. Less  than  1 per  cent,  of  pyrite  was  found,  the  re- 
mainder having  altered  into  chalcocite. 


Initial  Conditions  of  the  Experiment. 


| 

Solution. 

Exp. 

Material. 

Weight,  g. 

Quantity. 

Copper, 
Initial  g. 

12 

Leadville  pyrite, 

125—200  mesh 

0.5500 

75  C.c.  5 percent. 

C11SO4 . 5H2O 

O.8939 

The  enrichment  product  on  the  pyrite  was  removed  with  a 5 
per  cent,  solution  of  potassium  cyanide  and  analyzed  in  the 
manner  previously  described. 


Analysis  of  Enrichment  Product. 


Exp. 

Cu  in  KCN, 
g. 

S. in  KCN,  g. 

Total,  g. 

Per  Cent,  of 
Cu. 

Per  Cent,  of 
C112S  Based  on 
Per  Cent, 
of  Cu. 

Per  Cent,  of 
CuS. 

12 

0.3790 

0.0961 

0.4751 

79.96 

99-3 

0.7 

436 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


Thus  the  analysis  confirms  the  microscopic  observation  that  the 
pyrite  had  altered  to  chalcocite. 

The  solution  resulting  from  the  reaction  between  pyrite  and 
cupric  sulphate  was  analyzed  and  shown  to  contain  iron  in  the 
ferrous  condition,  sulphuric  acid,  and  unused  copper  as  sulphate. 


Analysis  of  the  Solution. 


Duration  of 

Copper,  g. 

Total  Iron 

H2S04,  g. 

Exp. 

Experiment. 

Initial. 

Final. 

Deposited. 

(Ferrous),  g. 

12 

9 days 

O.8939  1 

O.IOSO 

O.7889 

O.2521 

I.0250 

Unless  otherwise  noted,  in  all  of  our  experiments  the  filtrate 
from  the  residue  was  diluted  to  a measured  volume  and  aliquot 
portions  used  for  determining  the  various  constituents. 

The  ferrous  iron  was  determined  by  titration  with  potassium 
permanganate  in  the  usual  manner.  The  total  iron  was  deter- 
mined gravimetrically.  'When  copper  is  still  present  at  the  end 
of  an  experiment,  the  following  equilibrium  shows  that  the  fer- 
rous iron  formed  during  the  experiment  is  not  necessarily  the 
total  iron,  even  though  the  amount  of  ferrous  iron  obtained  by 
titration  with  KMn04  is  equal  to  the  total  iron  determined  gravi- 
metrically : 

Cu"  + Fe"  Cu'  + Fe'".30  (4) 

In  the  titration  with  permanganate,  the  cuprous  copper  as  well 
as  the  ferrous  iron  is  oxidized  by  the  permanganate.  The  amount 
used  for  this  titration  would  be  equivalent  to  the  total  iron  de- 
termined gravimetrically. 

It  has  been  shown,  however,  that  the  amounts  of  ferric  iron 
and  cuprous  copper  that  can  coexist  in  solution  must  be  very 
small  as  compared  with  the  amounts  of  ferrous  iron  and  cupric 
copper.31  Then,  too,  the  amount  of  copper  sulphate  left  in  the 
solution  at  the  end  of  an  experiment  was  usually  very  small. 
Consequently,  the  amount  of  ferric  iron  in  solution  must  have 
been  negligible  as  compared  with  the  amount  of  ferrous  iron. 

30  The  existence  of  this  equilibrium  was  first  suggested  by  H.  C.  Biddle. 
See  Am.  Chem.  J 26,  377,  1901. 

31  See  page  453,  footnote  46. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  437 


When  sufficient  copper  is  present  to  color  the  solution,  the 
titration  of  the  ferrous  iron  with  potassium  permanganate  and 
the  titration  of  the  acid  with  sodium  carbonate  are  rendered 
somewhat  difficult  owing  to  the  fact  that  the  color  of  the  copper 
in  solution  interferes  with  the  accurate  determination  of  the  end 
point  in  each  case.  The  errors  thus  introduced  can  be  rendered 
negligible,  if  the  color  of  the  solution  is  not  too  strong,  by  titrat- 
ing in  the  first  case  to  the  color  obtained  on  adding  a drop  of  per- 
manganate solution  to  a solution  of  cupric  sulphate  of  the  same 
concentration  as  that  which  obtains  at  the  end  of  the  experiment ; 
and  in  the  second  case,  to  the  color  obtained  on  adding  the  usual 
amount  of  methyl  orange  to  the  blank.  Finally,  the  titration 
of  the  acid  must  be  carried  out  in  the  presence  of  some  indiffer- 
ent gas;  otherwise  it  is  impossible  to  determine  the  end  point 
accurately,  owing  to  the  presence  of  ferrous  iron  which  on  ex- 
posure to  the  air  is  oxidized  and  hydrolyzed  very  rapidly  towards 
the  end  of  the  titration,  when  the  acid  is  sufficiently  diminished. 

Molecular  ratios. — The  following  molecular  ratios  were  deter- 
mined on  the  basis  of  the  above  analysis  of  the  solution: 


Cu  Deposited  H2SO4 

Fe  in  Solution"  . Fe 

2.75  2.32 


Cu  Deposited 
H2SO4  ‘ 

I.IQ 


When  the  proof  of  the  formation  of  cuprous  sulphide  both  by 
analysis  and  by  microscopic  observation  is  taken  together  with 
the  molecular  ratios,  it  is  obvious  that  the  alteration  of  pyrite  by 
cupric  sulphate  under  these  conditions  is  well  represented  by  the 
following  equation : 

5FeS2+i4CuS04+i2H20=7Cu2S+5FeS04+i2H2S04.  (5) 

The  molecular  ratios  demanded  by  this  equation  and  those 
obtained  experimentally  are  tabulated  below: 


Cu 

H2SO4 

Cu 

Fe* 

Fe  ' 

H2SO4' 

Demanded  by  equation  (5) 

2.80 

2.40 

I.I7- 

Found 

2-75 

2.32 

I.I9 

438 


E.  G.  Z1ES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


The  agreement  is  as  close  as  the  experimental  errors  peculiar 
to  the  method  of  the  investigation  will  permit. 

( b ) Alteration  of  Pyrite  to  Cupric  and  Cuprous  Sulphides 

at  200°. 

Numerous  experiments  were  carried  out  in  order  to  induce 
pyrite  to  alter  into  cupric  sulphide  but  up  to  the  present  time  we 
have  been  unable  to  obtain  cupric  sulphide  as  the  only  enrichment 
product;  both  cupric  and  cuprous  sulphide  were  invariably 
obtained.  The  initial  conditions  of  the  experiments  just  men- 
tioned are  tabulated  below.  Here,  as  before,  the  pyrite  was 
mixed  with  three  times  its  volume  of  pure  quartz.  The  Elba 
pyrite  mentioned  in  the  table  was  analyzed  for  copper  and  in- 
soluble residue  but  none  were  found.  Microscopical  examination 
revealed  the  presence  of  a minute  quantity  of  hematite. 


TABLE  IX. 

Pyrite  and  Cupric  Sulphate. 

Initial  Conditions  of  Experiments  at  200°. 


Exp. 

Material. 

Weight,  g. 

Solution. 

13 

Elba,  pyrite,  ioo  mesh  and  finer. 

1.0000 

50  c.c.  5 per  cent.  CUSO4.5H2O 

14 

it  a a 

* * 

“ “ “ 

15 

Leadville,  pyrite,  125-200  mesh  . 

40  c.c. 

16 

<<  it  it 

“ “ “ 

17 

u it  it 

“ “ “ 

18 

Elba,  pyrite,  100  mesh  and  finer. 

I5.000032 

42  c.c. 

The  enrichment  products  replacing  the  pyrite  were  analyzed  in 
the  manner  previously  described.  In  experiment  18  the  color  of 
the  residue  strongly  suggested  that  of  tarnished  bornite;  the 
microscope,  however,  revealed  that  the  color  was  a kind  of  com- 
posite color  and  was  due  to  the  iridescent  effect  caused  by  the 
thinness  of  the  film  of  alteration  products  on  the  pyrite  and  to 
the  color  of  the  pyrite  itself. 

32  In  order  to  facilitate  the  evacuation  of  the  tube  and  also  to  facilitate  cir- 
culation of  the  solution  through  this  large  mass  of  material,  an  ir>ner  tube  was 
used.  The  mixture  of  sulphide  and  quartz  was  distributed  around  this  tube, 
which  projected  above  the  mixture  about  i cm.  The  solution  was  introduced 
through  the  inner  tube,  care  being  taken  not  to  let  it  overflow  until  the  solu- 
tion came  through  to  the  top  of  the  quartz  and  sulphide. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


439 


TABLE  X. 

Pyrite  and  Cupric  Sulphate. 
Analyses  of  the  Enrichment  Products. 


Exp. 

Cu  in  KCN,  g. 

S in  KCN,  g. 

Total,  g. 

Per  Cent, 
of  Cu. 

Per  Cent, 
of  Cu2S. 

Per  Cent, 
of  CuS 

by  Difference. 

13 

0.6352s3 

0.1800 

0.8152 

77.92 

86  ± I % 

14 

14 

0.577033 

O.1645 

0.7415 

77.82 

85  “ 

15 

17 

0.1478 

O.O420 

0.1898 

77.87 

85  “ 

15 

18 

O.1995 

O.0577 

O.2572 

77.60 

83  “ 

17 

The  residues  in  13,  14,  and  17  were  compressed  and  examined 
microscopically.  This  examination  showed  that  a large  percent- 
age of  the  pyrite  was  still  unattacked  and  also  that  both  cuprous 
and  cupric  sulphides  were  present  as  alteration  products  on  the 
pyrite.  The  film  of  alteration  products  formed  on  the  pyrite  in 
18  was  too  thin  to  permit  a similar  microscopic  examination. 

The  analyses  of  the  enrichment  products  show  that  in  13,  14, 
and  17  the  percentage  of  cupric  and  cuprous  sulphides  remained 
fairly  constant  but  changed  several  per  cent,  in  18,  where  the 
amount  and  surface  of  the  pyrite  were  much  greater  than  in  the 
other  experiments. 

Analyses  of  the  Solutions. — The  filtrates  from  the  residues 
were  analyzed  in  the  usual  manner.  These  analyses,  together 
with  the  molecular  ratios  based  on  them,  are  tabulated  below. 


TABLE  XI. 

Pyrite  and  Cupric  Sulphate. 
Temperature , 200°. 


Exp. 

Dura- 

tion, 

Days. 

Analyses  of  Solutions. 

Molecular  Ratios  on  the  Basis  of 
the  Analyses. 

Copper,  g. 

Total 
Iron  as 
Ferrous, 
g ■ 

Acid,  g. 

h2so*. 

Cu  Deposited 

h2so4 

Cu 

HjSO*' 

Initial. 

Final. 

Depos- 

ited. 

Fe  in  Solution' 

Fe  ' 

13 

9 

O.642O 

None 

0.6420 

0.2205 

Not  det. 

2.56 

34 

34 

14 

8 

O.6424 

None 

O.6424 

0.2209 

“ 

2.56 

34 

34 

15 

20 

0.5m 

None 

O.511I 

O.I728 

0.6600 

2.60 

2.18 

1.20 

16 

8 

0.5084 

0.0255 

O.4829 

0.1630 

0.6225  I 

2.66 

2.17 

1.23 

17 

8 

0.5084 

0.0113 

O.4971 

O.1696 

0.6392 

2-57 

2.15 

1.20 

18 

2 

0.5405 

| None 

l 0.5405 

O.1838 

Not  det.  i 

2.58 

34 

34 

33  Both  copper  and  sulphur  were  determined  ill  aliquot  portions  of  the  solu- 
tion of  copper  sulphides  in  potassium  cyanide. 

34H2S04  not  determined  since  Jena  glass  reaction  tubes  were  used. 


440 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


Discussion. — Experiments  13  to  17  were  carried  out  under 
similar  conditions,  namely,  the  same  amount  of  pyrite  was  ex- 
posed to  the  action  of  a 5 per  cent,  solution  of  cupric  sulphate; 
likewise  a large  percentage  of  pyrite  was  unattacked.  The  mo- 
lecular ratios  obtained  in  these  experiments  show  that  the  condi- 
tions at  the  end  of  the  experiments  must  have  been  nearly  the 
same.  Now  the  analyses  of  the  alteration  products  and  their 
microscopic  examination  proved  the  presence  of  both  cuprous  and 
cupric  sulphides,  and  the  uniformity  of  the  conditions  at  the  end 
of  the  experiment  was  again  shown  by  the  fact  that  the  per- 
centages of  the  two  sulphides  of  copper  were  about  the  same. 
The  following  hypothetical  equation  has  been  suggested  to  repre- 
sent the  alteration  of  pyrite  into  cupric  sulphide. 

4FeS2  + 7CuS04  + 4H20  = 7CuS  + 4FeS04  + 4H2S04.  (6) 

The  molecular  ratios  demanded  by  this  equation  and  by  equation 
(5),  the  correctness  of  which  we  have  proven,  are  tabulated 
below : 


Equation. 

Cu  J 

h2so4 

Cu 

fT 

Fe 

H2SO4 

(5) 

2.80 

2.40 

I,I7 

(6) 

i-75 

1. 00 

i-75 

The  ratios  obtained  in  experiments  13  and  18  all  lie  between  these 
values.  This  evidence,  together  with  that  furnished  by  the  anal- 
yses of  the  alteration  products  and  by  their  microscopical  exami- 
nation, serves  at  least  to  indicate  that  the  reaction  represented  by 
equation  (6)  is  involved  when  cupric  sulphide  is  obtained  as  one 
of  the  enrichment  products  replacing  pyrite.  We  have  proven 
that  covellite  reacts  with  cupric  sulphate  to  form  cuprous  sul- 
phide, and  we  have  shown  that  this  reaction  takes  place  readily 
at  200°.  This  being  the  case,  the  evidence  seems  good  that  when 
pyrite  unde'r  the  conditions  of  our  experiments  alters  into  cuprous 
sulphide,  both  reactions  just  indicated  are  involved,  thus : 

4FeS2  + 7CuS04  + 4H20  = 7CuS  + 4FeS04  + 4H2S04.  (6) 

5Q1S  + 3CuS04  + 4H20  = 4Cu2S  + 4H2S04.  ( 1 ) 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


441 


According  to  these  equations,  all  of  the  iron  in  solution  would  be 
derived  through  (6),  but  the  sulphuric  acid  would  be  derived 
through  both  reactions.  Now,  if  we  know  the  total  amount  of 
copper  which  took  part  in  the  reaction,  and  also  the  total  iron  in 
solution,  we  should,  on  the  basis  of  these  equations,  be  able  to 
calculate  the  amount  of  cuprous  and  cupric  sulphides  present  in 
the  alteration  products  on  the  pyrite.  We  should  also  be  able  to 
determine  both  sulphides,  knowing  the  amount  of  total  copper 
which  reacted  and  the  amount  of  sulphuric  acid  formed.  These 
calculations  were  made  for  experiment  17,  Table  X,  and  the 
values  thus  found  compared  with  those  obtained  by  analysis  of 
the  alteration  product  on  the  pyrite: 


Per  Cent.  C112S. 


Based  on  analysis  85 

Based  on  Cu  and  Fe 83 

Based  on  Cu  and  H2S04  87 


Per  Cent.  CuS. 

15 
1 7 
13 


The  agreement  is  as  close  as  could  be  expected  when  all  of  the 
experimental  errors  involved  are  considered.  The  same  calcula- 
tions were  made  in  the  case  of  experiments  12,  13,  and  17,  and 
similar  agreement  was  found  between  the  values  thus  obtained 
and  those  found  by  analysis  of  the  alteration  product. 

The  percentages  of  CuS  and  Cu2S,  shown  in  Table  XII., 
indicate  that  reaction  (1)  proceeds  faster  than  (6).  The  relative 
rates  of  the  two  reactions  under  the  conditions  of  our  experi- 
ments are  in  all  probability  influenced  by  the  extent  of  the  altera- 
tion of  the  pyrite.  Thus  as  the  thickness  of  the  enveloping  coat- 
ing of  the  sulphide  of  copper  which  replaces  the  pyrite  increases, 
there  will  be  greater  opportunity  for  reaction  (1)  to  take  place 
than  for  (6),  due  to  the  fact  that  the  enrichment  products  replac- 
ing the  pyrite  prevent  ready  access  of  the  cupric  sulphate  solu- 
tion. Consequently,  as  the  reacting  surface  of  the  pyrite  is  in- 
creased, for  a given  concentration  of  copper,  more  cupric  sul- 
phide should  be  formed.  This  is  indicated  at  least  in  experi- 
ment 17,  where  the  amount  of  cupric  sulphide  is  several  per  cent, 
greater  than  in  the  other  experiments,  and  probably  approaches 


442 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


more  closely  the  relative  rates  of  the  two  reactions  involved  if 
the  copper  enrichment  products  derived  from  the  pyrite  were  to 
remain  detached  from  the  pyrite. 

We  do  not  wish  to  leave  the  impression  that  all  of  the  cupric 
sulphide  is  formed  at  one  time,  and  that  this  is  then  further  at- 
tacked by  the  cupric  sulphate;  it  is  entirely  possible  that  the  two 
reactions  go  on  simultaneously,  but  when  the  pyrite  is  covered 
with  its  alteration  products,  the  opportunity  for  reaction  between 
cupric  sulphide  and  cupric  sulphate  is  more  favorable  than  before. 


(c)  The  Influence  of  Sulphuric  Acid  on  the  Reaction  between 
Pyrite  and  Cupric  Sulphate  at  200°. 

The  following  experiments  were  carried  out  in  order  to  de- 
termine in  what  manner  sulphuric  acid  influenced  the  reaction 
between  pyrite  and  cupric  sulphate. 

TABLE  XII. 

Influence  of  Sulphuric  Acid. 

Initial  Conditions  of  the  Experiments  ( Temperature , 200°). 


Exp. 

Material. 

Weight, 

g . 

Solution. 

Group  I . . ! 

19 

Elba  pyrite,  ioo  mesh  and 
finer. 

I. OOOO 

50  c.c.  Cupric  sulphate,  5% 
and  acid,  5%. 

20 

Elba  pyrite,  ioo  mesh  and 
finer. 

1. 0000 

50  c.c.  Cupric  sulphate,  5% 
no  acid. 

Group  II . 

21 

Leadville  pyrite,  125-200 
mesh. 

1. 0000 

40  c.c.  Cupric  sulphate,  5% 
and  acid,  2 %. 

22 

Leadville  pyrite,  125-200 
mesh. 

1. 0000 

40  c.c.  Cupric  sulphate,  5% 
no  acid. 

23 

Leadville  pyrite,  125-200 
mesh. 

1. 0000 

40  c.c.  Cupric  sulphate,  5% 
no  acid. 

Group  III 

24 

Leadville  pyrite,  125-200 
mesh. 

1. 0000 

50  c.c.  Cupric  sulphate,  5% 
and  acid,  1!%. 

25 

Leadville  pyrite,  125-200 
mesh. 

1. 0000 

! 50  c.c.  Cupric  sulphate,  5% 
no  acid. 

The  solutions  and  residues  obtained  in  these  experiments  were 
analyzed  in  the  same  manner  as  in  the  previous  experiments  on 
pyrite.  Each  group  of  experiments  is  to  be  considered  sepa- 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  443 


rately,  since  the  experiments  in  each  group  were  carried  out  under 
comparable  conditions,  though  it  should  be  remembered  that  in 
the  second  and  third  groups  the  tubes  in  which  acid  was  used 
were  exposed  to  the  temperature  of  200 ° for  a longer  period  than 
those  in  which  no  acid  was  used. 

The  solutions  obtained  in  these  experiments*  analyzed  as 
follows : 

TABLE  XIII. 

Influence  of  Sulphuric  Acid. 


Group. 

Exp. 

Duration. 

Analyses  of  the  Solutions. 

Initial. 

Copper,  g. 

Final. 

Deposited. 

Acid,  g. 

Initial. 

I 

19 

5 days 

O.6325 

O.5568 

0.0757 

2.4500,  5% 

20 

5 “ 

O.6325 

O.0963 

O.5362 

None 

II 

21 

15  “ 

O.5111 

O.IO65 

O.4046 

0.8000,  2% 

22 

8 “ 

0.5084 

0.0255 

O.4829 

None 

23 

8 “ 

0.5084 

O.OI33 

O.4951 

None 

III.  . . 

24 

7 " 

O.6366 

O.1846 

0.4521 

0.7500,  \\% 

25 

4 “ 

O.6366 

O.0273 

0.5093 

None 

On  comparing  19  and  20  in  Group  I.,  we  find  a marked  differ- 
ence in  the  amount  of  copper  deposited,  the  duration  of  the  ex- 
periment being  the  same  in  both  cases,  a fact  which  indicates  a 
distinct  retarding  influence  on  the  part  of  the  acid.  In  the  second 
and  third  group  of  experiments,  a similar  retarding  influence  is 
noted,  even  though  the  tubes  containing  the  acid  were  heated  for 
a longer  period  of  time  than  those  in  which  no  acid  was  used 
initially.  In  experiment  21,  Group  II.,  the  tube  was  removed 
from  the  furnace  at  the  end  of  eight  days  but  the  appearance 
of  the  pyrite  and  that  of  the  solution  indicated  that  very  little 
action  had  taken  place.  Thereafter  the  tube  was  removed  from 
the  furnace  daily  for  examination  until  the  end  of  the  experiment, 
and  it  was  noted  that  the  solution  of  cupric  sulphate  gradually 
lost  its  characteristic  color,  indicating  that  reaction  was  taking 
place  and  that  the  acid  exerted  a retarding  influence  but  did  not 
inhibit  the  reaction,  as  would  have  been  the  case  if  it  were  a 


444 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


question  of  increased  solubility  of  one  of  the  sulphides  due  to  the 
presence  of  a great  excess  of  hydrogen  ions.  This  inhibiting 
effect  of  the  acid  is  probably  due,  in  part  at  least,  to  another  cause, 
to  which  we  shall  refer  subsequently. 

Analyses  of  Enrichment  Products. — The  enrichment  products 
which  replace  the  pyrite,  both  when  acid  was  used  initially  and 
when  no  acid  was  used  initially,  were  analyzed  in  the  usual 
manner. 

TABLE  XIV. 

Influence  of  Sulphuric  Acid. 


Group. 

Exp. 

Analyses  of  the  Enrichment  Products. 

Cuin  KCN, 
S • 

S in  KCN, 
g • 

Total, 
g • 

Per  Cent, 
of  Cu. 

Per  Cent, 
of  CU2S. 

Per  Cent, 
of  CuS  by 
Difference. 

I 

19 

acid 

0.0763 

0.0225 

O.O988 

77-23 

80  ± 3% 

20 

20 

none 

0.1442 

0.0414 

O.1856 

77.69 

84  ± I % 

16 

II 

21 

acid 

0.1093 

0.0311 

O.1404 

77-85 

85  ± 1% 

15 

23 

none 

0.1478 

0.0420 

O.1898 

77.87 

85  ±1% 

15 

III.  . . 

24 

acid 

0.1330 

0.0384 

O.1714 

77-59 

83  ±1% 

17 

25 

none 

0.2010 

0.0570 

6.2580 

77-90 

85  ± 1% 

15 

On  comparing  the  analyses  in  each  group,  we  find  that,  within 
the  limits  of  the  experimental  error,  the  acid  has  not  changed 
the  relative  amounts  of  cupric  and  cuprous  sulphides;  therefore 
the  acid  has  exerted  only  a retarding  influence  and  has  not 
changed  the  nature  of  the  reaction  between  pyrite  and  cupric 
sulphate. 

Experiment  26.  Action  of  Sulphuric  Acid  on  Pyrite. — We 
find  that  pyrite  is  only  very  slightly  attacked  at  200°  by  a 2 per 
cent,  solution  of  sulphuric  acid,  as  the  following  will  show : 1 g. 
of  Leadville  pyrite,  sized  as  usual  between  125  and  200  mesh,35 
and  mixed  with  three  times  its  volume  of  quartz,  was  exposed  at 
200°  for  a period  of  three  days  to  the  action  of  a 2 per  cent, 
solution  of  sulphuric  acid,  and  the  amount  of  hydrogen  sulphide 
in  the  tube  determined.  The  tubes  were  opened  without  expos- 
ing their  contents  to  the  air,  in  the  manner  described  on  page  418. 

35  The  pyrite  was  carefully  sized  in  order  to  compare  the  reactivity  of  pyrite 
and  chalcopyrite  toward  sulphuric  acid. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  445 


The  hydrogen  sulphide  was  driven  out  by  means  of  hydrogen  gas 
and  absorbed  in  cadmium  acetate.  Under  these  conditions  0.6 
milligram  of  hydrogen  sulphide  and  4 milligrams  of  iron  as 
sulphate  were  found.  These  small  amounts  certainly  show  that 
pyrite  is  not  readily  attacked  by  sulphuric  acid.  The  tube  in 
which  this  experiment  was  carried  out  was  very  carefully  evacu- 
ated before  sealing,  therefore  no  oxidation  due  to  the  presence  of 
air  in  the  tube  could  have  taken  place.  As  a matter  of  fact,  we 
have  just  shown  that  sulphuric  acid  exerts  a markedly  retarding 
influence  on  the  reaction  between  pyrite  and  cupric  sulphate. 
This  certainly  could  not  be  the  case  if  pyrite  were  readily  attacked 
by  sulphuric  acid  with  the  formation  of  hydrogen  sulphide. 

(1)  Secondary  Reactions . 

It  will  be  remembered  that  in  our  preliminary  experiments  on 
the  reaction  between  pyrite  and  cupric  sulphate,  hematite,  cuprite, 
and  metallic  copper  were  formed  together  with  the  sulphides  of 
copper,  thus  confirming  a similar  observation  by  Stokes.36  It 
will  also  be  remembered  that  we  succeeded  in  establishing  such 
experimental  conditions  that  at  the  end  of  an  experiment  only  the 
sulphides  of  copper  were  found  as  alteration  products  on  the 
pyrite.  While  this  was  the  case  at  the  end  of  any  one  experiment, 
in  its  earlier  stages  hematite,  cuprite,  and  metallic  copper  were 
also  formed.  The  question  now  arises,  how  were  these  three  sub- 
stances formed  and  why  did  they  disappear. 

When  pyrite  alters  into  the  sulphides  of  copper,  ferrous  sul- 
phate is  formed  as  one  of  the  soluble  products.  Stokes36  showed 
that  ferrous  sulphate  and  cupric  sulphate  react  at  200°  to  form 
hematite,  cuprite,  and  metallic  copper,  and  showed  that  their 
presence  could  be  explained  by  the  following  equilibrium : 

2FeS04  -f-  2CuS04  ?=*  Cu2S04  + Fe2(S04)3. 

When  no  acid  is  used  as  an  initial  constituent  of  the  solution, 
the  ferric  sulphate  so  formed  will  be,  at  200°,  largely  hydrolyzed, 

36  H.  N.  Stokes,  Bull.  U.  S.  Geol.  Survey,  No.  186,  p.  44. 


446  E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN.  . 

and  the  iron  will  be  precipitated  as  hematite,  thus  permitting  the 
equilibrium  to  shift  more  and  more  to  the  right  until  the  acid 
formed  as  the  result  of  the  hydrolysis  of  the  ferric  sulphate  is 
in  equilibrium  with  the  precipitated  hematite.  This  shift  of  the 
equilibrium  brings  the  concentration  of  the  cuprous  sulphate  to  a 
point  where  it  also  hydrolyzes,  yielding  cuprite  and  sulphuric  acid. 
Just  as  with  ferric  sulphate,  this  hydrolysis  will  go  on  until  the 
cuprite  and  acid  are  in  equilibrium. 

The  formation  of  the  metallic  copper  is  easily  explained  by 
another  well-known  reaction  of  cuprous  sulphate : 

Cu  + CuS04  ^ Cu2S04. 

This  reaction  is  readily  reversible,  and  the  amount  of  copper  pre- 
cipitated at  any  temperature  will  depend  upon  the  concentrations 
of  cuprous  and  cupric  copper.  It  is  therefore  obvious  that  the 
addition  of  sufficient  sulphuric  acid  to  a reaction  mixture  con- 
sisting of  copper  and  iron  sulphates  will,  by  preventing  the 
hydrolysis  of  one  of  the  products,  ferric  sulphate  for  instance, 
prevent  the  formation  of  hematite,  cuprite,  and  metallic  copper. 

Now  let  us  consider  what  happens  when  pyrite  and  cupric  sul- 
phate react.  We  have  shown  that  in  this  reaction  ferrous  sul- 
phate and  sulphuric  acid  are  formed,  and  that,  depending  on  the 
conditions  of  the  experiment,  pyrite  alters  into  cuprous  sulphide, 
or  into  cuprous  and  cupric  sulphides.  In  the  first  stage  of  the 
experiment,  we  have  present  a small  amount  of  ferrous  sulphate 
and  sulphuric  acid  and  a relatively  large  amount  of  cupric  sul- 
phate; in  consequence  of  the  reaction  between  ferrous  sulphate 
and  cupric  sulphate  explained  above,  hematite,  cuprite,  and  metal- 
lic copper  will  form  until  all  three  are  in  equilibrium  with  the  sul- 
phuric acid  present.  In  the  later  stages  of  the  reaction  between 
cupric  sulphate  and  pyrite,  enough  acid  is  formed  to  redissolve 
both  hematite  and  cuprite,  and  it  will  be  evident  from  what  has 
just  been  said  that  the  ferric  sulphate  formed  by  the  solution  of 
the  hematite  must  be  reduced  both  by  metallic  copper  and  by 
the  cuprous  sulphate  formed  by  the  solution  of  the  cuprite. 

One  can  readily  see,  therefore,  that  in  the  reaction  between 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  44 7 

pyrite  and  cupric  sulphate,  the  more  rapidly  the  copper  in  solu- 
tion is  used  up,  the  more  rapidly  sufficient  sulphuric  acid  will  be 
formed  to  prevent  the  hydrolysis  of  the  ferric  sulphate  resulting 
from  the  reaction  between  ferrous  and  cupric  sulphates.  The 
copper  can  be  used  up  rapidly  by  presenting  a large  surface  of  the 
pyrite  to  the  action  of  the  solution;  thus,  after  carrying  out  a 
large  number  of  preliminary  experiments,  it  was  found  that  at 
200°  one  gram  of  ioo-mesh  pyrite  mixed  with  three  times  its 
volume  of  quartz  will  use  up  in  eight  days  all  of  the  copper  in 
50  c.c.  of  a 5 per  cent.  CuS04.5H20  solution,  and  no  hematite, 
cuprite,  or  metallic  copper  will  be  present  at  the  end  of  the  experi- 
ment. When  no  quartz  was  mixed  with  the  pyrite,  several  weeks 
were  required  to  accomplish  the  same  result.  An  examination 
of  Tables  XI.  and  XIII.  will  show  that,  as  the  reacting  surface  of 
the  pyrite  was  increased  and  made  as  available  as  possible  by 
admixing  with  quartz,  the  time  required  for  the  pyrite  to  use 
up  all  or  nearly  all  of  the  copper  in  solution  and  leave  no  hema- 
tite, cuprite,  or  metallic  copper  at  the  end  of  the  experiment  is 
greatly  decreased. 

The  formation  of  these  secondary  products  can  also  be  avoided 
if  sulphuric  acid  of  the  proper  concentration  is  present  as  one  of 
the  initial  constituents  of  the  solution.  We  have  shown,  how- 
ever, that  sulphuric  acid  retards  the  reaction  between  pyrite  and 
cupric  sulphate  but  does  not  change  the  nature  of  the  reaction. 

We  believe  that  the  retarding  influence  of  the  acid  is  due  to 
its  maintaining  in  solution  the  ferric  iron  formed  in  the  reaction 
between  cupric  and  ferrous  sulphates.  When  no  sulphuric  acid  is 
used  initially,  we  have  the  oxidizing  action  of  the  cupric  sul- 
phate augmented  by  the  action  of  the  cuprous  sulphate  formed 
in  the  reaction  between  cupric  and  ferrous  sulphates.37  As  the 
acid  increases,  the  ferric  iron  is  maintained  in  solution  and  the 
augmenting  action  of  this  cuprous  sulphate  is  rendered  negligible. 
When  sulphuric  acid  is  used  as  one  of  the  initial  constituents  of 
the  solution,  the  ferric  iron  is  maintained  in  solution  but  the 
amount  at  the  end  of  the  experiment  is  very  small.  (See  page 
437)- 

37  See  page  446. 


448 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


( d ) The  Influence  of  Ferrous  Sulphate  on  the  Reaction  between 
Pyrite  and  Cupric  Sulphate  at  200° . 

It  has  been  thought  by  several  investigators  that  ferrous  sul- 
phate plays  a very  important  part  in  the  reaction  between  pyrite 
and  cupric  sulphate.  We  tried  to  obtain  information  on  this 
point  at  200°,  but  were  compelled  to  discontinue  the  experimental 
work  for  the  following  reason:  When  ferrous  sulphate  is  used  as 
an  initial  constituent  of  the  solution,  the  troublesome  secondary 
reactions  discussed  on  page  446  become  more  troublesome  than 
ever;  if  sulphuric  acid  is  added  to  overcome  the  hydrolysis  of 
the  ferric  sulphate  and  cuprous  sulphates,  the  retarding  influence 
of  the  acid  which  we  have  previously  described  comes  into  play, 
consequently  nothing  could  be  gained  by  carrying  out  extensive 
experiments  along  these  lines. 

B.  The  Reaction  between  Pyrite  and  Cupric  Sulphate  at  ioo°. 

The  experiments  at  this  temperature  were  carried  out  as  fol- 
lows : One  gram  of  pyrite  was  mixed  with  three  times  its  volume 
of  pure  quartz  and  exposed  to  the  action  of  a 1 per  cent,  cupric 
sulphate  solution.  Under  these  conditions  no  hematite  was  found 
at  the  end  of  the  experiment.  At  the  same  time  experiments  were 
carried  out  in  order  to  study  the  influence  of  sulphuric  acid,  and 
also  that  of  ferrous  sulphate,  on  the  reaction  between  pyrite  and 
cupric  sulphate. 

TABLE  XV. 

Pyrite  and  Cupric  Sulphate. 

Initial  Conditions:  Weight  of  Leadville  pyrite  (200-220  mesh),  1.0000  g. ; 

temperature,  ioo°. 


Exp. 

Dura- 

tion, 

Months. 

Initial  Concentrations  of  Solutions. 

Analyses  of  Soli 

Copper,  g. 

itions. 

Per 

Cent. 

Acid, 

Initial. 

Per  Cent. 
Ferrous 
Iron, 
Initial. 

Initial. 

Final.  1 

Deposited. 

27 

2 

50  C.C.  1%  CUSO4.5H2O 

0.1286 

0.II50 

O.OI36 

None 

None 

28 

2 

“ “ “ 

0.1286 

O.II60 

0.0126 

“ 

29 

2 

50  c.c.  1%  CUSO4.5H2O,  and 

1%  H2SO4 

0.1286 

O.1230 

O.OO56 

1% 

il 

30 

2 

50  C.C.  1%  CUSO4.5H2O, 

1%  FeSC>4.7H20,  and 

1 % H2SO4 

0.1286 

Q-I233 

0.0053 

1% 

1% 

SECONDARY  COPPER  SULPHIDE  ENRICHMENT , 449 


It  was  necessary  to  determine  the  ferrous  iron  and  sulphuric 
acid  formed  during  the  experiment  in  aliquot  portions  of  the 
solution,  and  the  amounts  found  were  too  small  and  the  experi- 
mental errors  too  large  to  permit  their  being  determined  with 
accuracy.  The  copper,  however,  could  be  accurately  determined 
and  the  determinations  are  believed  to  be  correct  to  within  0.3 
milligram  of  copper.  The  pyrite  in  27  and  28  was  completely 
covered  with  a grayish  blue  alteration  product;  the  pyrite  in  29 
and  30  was  barely  tarnished,  indicating  that  very  slight  action  had 
taken  place.  On  comparing  the  amount  of  copper  deposited  in 
29  with  the  amounts  deposited  in  27  and  28,  one  can  readily  see 
that  the  acid  has  also  at  ioo°  exerted  a retarding  influence  on  the 
reaction  between  pyrite  and  cupric  sulphate;  the  agreement  be- 
tween the  amounts  of  copper  deposited  in  27  and  28  shows  that 
the  difference  observed  between  the  amounts  of  copper  deposited 
in  29  and  the  amounts  deposited  in  27  and  28  is  too  great  to  be 
attributed  to  experimental  error ; the  difference  must  be  attributed 
to  the  retarding  influence  exerted  by  the  acid.  On  comparing  29 
and  30  we  find  that  within  the  limits  of  the  experimental  error 
the  same  amounts  of  copper  were  deposited.  It  will  be  noted 
that  sulphuric  acid  was  present  along  with  the  ferrous  sulphate; 
this  was  added  to  prevent  the  hydrolysis  of  the  ferric  sulphate 
formed  in  the  reaction  between  ferrous  sulphate  and  cupric  sul- 
phate; it  is  therefore  evident  that  when  sulphuric  acid  is  present, 
ferrous  sulphate  exerts  no  influence  on  the  reaction  between  pyrite 
and  cupric  sulphate. 

The  enrichment  product  replacing  the  pyrite  in  experiment  28 
was  removed  with  a 1 per  cent,  solution  of  potassium  cyanide, 
and  analyzed : 


Analysis  of  the  Enrichment  Product. 


Exp. 

Copper  in 
KCN,  g. 

S in  KCN,  g. 

Total,  g. 

Per  Cent,  of 
Cu. 

Per  Cent,  of 
C112S. 

Per  Cent,  of 
CuS. 

28 

O.OO82 

O.OO25 

0.0107 

77 

76  ± 8 

24 

In  view  of  the  small  amounts  of  copper  and  sulphur  found  in 
the  KCN  solution,  the  percentages  of  Cu2S  and  CuS  shown 


450 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


above  can  be  considered  only  as  an  approximation.  We  made  a 
mixture  of  pure  chalcocite  and  covellite  containing  the  above  per- 
centage of  Cu2S  and  CuS,  and  dissolved  it  in  KCN,  using  a por- 
tion containing  about  the  same  amount  of  copper  and  sulphur  as 
indicated  in  the  analysis,  and  found  on  the  basis  of  the  result  that 
the  percentages  of  Cu2S  and  CuS  found  above  are  correct  only  to 
within  about  8 per  cent,  of  Cu2S  or  CuS.  This  accuracy  is  suffi- 
cient, however,  to  prove  that  both  Cu2S  and  CuS  were  present. 

Discussion. — Thus  pyrite  and  cupric  sulphate  will  react  also  at 
ioo°  and  yield  the  same  end  products  as  those  obtained  at  200°, 
namely,  cupric  and  cuprous  sulphides;  and  just  as  at  200 °,  sul- 
phuric acid  exerts  a retarding  influence  on  the  reaction.  Ferrous 
sulphate  is  apparently  without  effect  when  sulphuric  acid  is 
present. 

C.  Pyrite  and  Cupric  Sulphate  at  40°-^o°. 

Experiments  carried  out  by  Sullivan  indicated  that  pyrite  and 
cupric  sulphate  react  also  at  ordinary  temperatures.38  He  found 
that  20  grams  of  finely  divided  pyrite  caused  a solution  of  cupric 
sulphate  which  originally  contained  0.097  g.  Cu  in  40  c.c.  to  lose 
in  three  days  0.040  g.  Cu ; thus  indicating  that  pyrite  reacts  com- 
paratively easily  with  cupric  sulphate  at  ordinary  temperatures. 

We  desired,  if  possible,  to  obtain  additional  evidence  on  this 
point. 

Experiment  31. — At  first  15  grams  of  Elba39  pyrite  sized 
between  125-200  mesh  to  the  linear  inch  was  exposed  to  the 
action  of  a solution  of  cupric  sulphate  which  contained  0.3195  g. 
Cu  in  100  c.c.  of  solution.  The  experiment  was  carried  out  at 
40°  and  extended  over  a period  of  two  months.  The  containers 
in  this  and  in  the  subsequent  experiment  were,  as  usual,  care- 
fully evacuated  and  placed  in  the  shaking  machine  described  on 
page  415.  Only  2 milligrams  of  copper  were  lost  by  the  solution. 

Experiment  32. — Another  experiment  was  carried  out  this  time 
at  50°  in  the  following  manner:  15  grams  of  pyrite  finer  than 
200  mesh  were  exposed  to  the  action  of  200  c.c.  of  a copper  sul- 

38  E.  C.  Sullivan,  Trans.  Am.  Inst.  Min.  Eng.,  37,  894  (1907).  Unfortu- 
nately no  analysis  of  the  pyrite  used  in  this  experiment  is  mentioned. 

39  See  page  438  for  analysis. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


451 


phate  solution  of  the  same  strength  as  used  in  the  experiment 
above.  In  a period  of  six  months,  0.0523  g.  Cu  reacted  with 
the  pyrite.  The-  resulting  solution  was  decidedly  acid  and  con- 
tained about  0.052  g.  Fe  as  sulphate.  The  acid  and  iron  were 
not  accurately  determined  since  in  a previous  experiment  it  was 
found  that  on  washing  the  very  finely  divided  pyrite,  oxidation 
of  the  pyrite  by  the  air  caused  iron,  as  sulphate,  to  pass  into 
solution.40  Therefore  on  washing  the  cupric  sulphate  out  of  the 
pyrite,  iron,  as  sulphate,  passed  into  solution  in  addition  to  the 
iron  that  was  derived  from  the  reaction  between  pyrite  and  cupric 
sulphate. 

The  general  appearance  of  the  pyrite  had  not  changed  in  any 
marked  way,  but  on  washing  the  pyrite  with  1 per  cent.  KCN  and 
determining  the  copper  in  solution,  0.0520  g.  Cu  were  found.  On 
adding  a small  amount  of  nitric  acid  to  the  KCN  solution  the 
usual  precipitation  of  brownish  copper  sulphide  was  noted.  The 
sulphur  in  the  copper  sulphide  dissolved  by  the  KCN  solution 
could  not  be  accurately  determined  since  the  presence  of  consider- 
able iron  as  sulphate  was  noted  along  with  copper.  The  iron  was 
derived  by  oxidation  of  the  finely  divided  pyrite  in  the  same 
manner  as  above  indicated.  The  determination  of  the  sulphur 
could  not  be  satisfactorily  corrected  by  the  running  of  a blank  for 
the  correction  was  uncertain  and  too  large  to  permit  its  being 
properly  applied. 

The  results  show  that  pyrite  and  cupric  sulphate  react,  slowly41 
to  be  sure,  at  a temperature  much  below  ioo°  and  that  the  prod- 
ucts of  the  reaction  are  the  same  as  at  the  more  elevated  tempera- 
tures, namely,  sulphides  of  copper,  sulphuric  acid  and  iron 
sulphate. 

5.  THE  ACTION  OF  CUPROUS  SULPHATE  ON  PYRITE. 

It  is  evident  that  the  reaction  between  pure  pyrite  and  cupric 
sulphate  is,  at  lower  temperatures,  an  exceedingly  slow  one.  On 

40  Our  apparatus  for  filtering  out  of  contact  with  air  was  so  badly  clogged 
up  by  this  large  amount  of  finely  divided  pyrite  that  we  were -compelled  to 
carry  out  the  filtration  in  the  usual  manner. 

41  The  more  rapid  rate  indicated  by  Sullivan’s  experiments  is  possibly  due 
to  his  having  used  a pyrite  ground  more  finely  than  that  used  by  us. 


452 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


the  other  hand,  the  evidence  is  good  that  cuprous  sulphate  and 
pyrite  react  far  more  readily.  Thus  Winchell42  has  shown  that 
pyrite  exposed  to  the  action  of  cupric  sulphate  and  sulphur  diox- 
ide alters  rapidly  into  copper  sulphide  at  ordinary  temperatures, 
even  in  the  presence  of  sulphuric  acid.  Spencer43  has  shown 
that  when  pyrite  in  contact44  with  metallic  iron  is  treated  at 
ordinary  temperatures  with  cupric  sulphate,  it  is  coated  over  in  a 
comparatively  short  time  with  copper  sulphide  enrichment  prod- 
ucts. In  both  experiments  cuprous  copper  is  present  in  the  solu- 
tions; in  the  first  experiment  the  reduction  is  caused  by  the  sul- 
phur dioxide  and  in  the  second  by  the  metallic  iron. 

Inasmuch  as  cuprous  copper  can  play  such  an  important  part 
in  altering  pyrite  into  copper  sulphides,  we  should,  on  the  basis 
of  the  following  equilibrium,  expect  a similar  alteration  when 
pyrite  is  exposed  to  the  action  of  cupric  and  ferrous  sulphate,  be- 
cause ferrous  iron  partially  reduces  cupric  copper  to  cuprous 
copper,  as  explained  on  page  446. 

2Q1SO4  -f  2FeS04^Cu2S04  + Fe2(S04)3.45  (4) 

This  equilibrium  has  been  by  no  means  fully  investigated,  but 
we  feel  certain  that  the  quantities  of  ferric  iron  and  cuprous 
copper  that  can  coexist  in  solution  are  very  small  as  compared 
with  the  quantities  of  ferrous  iron  and  cupric  copper.46  Of 
course,  if  the  cuprous  copper  were  partially  removed  by  pre- 
cipitation, more  would  form  by  reaction  from  left  to  right  (see 
equation  (4)),  but  in  this  reaction  ferric  iron  must  also  form. 
Thus,  as  cuprous  copper  is  precipitated,  the  ferric  iron  continually 
increases,  which  means  that  the  concentration  of  cuprous  copper 
must  continually  decrease.  If  the  removal  of  the  ferric  iron 
could  proceed  pari  passu  with  the  precipitation  of  cuprous  copper, 

42  H.  V.  Winchell,  Bull.  Geol.  Soc.  Am.,  14,  269-276,  1903. 

43  A.  C.  Spencer,  Econ.  Geol.,  8,  629. 

44  It  is  not  necessary  for  the  metallic  iron  to  be  in  contact  with  the  pyrite, 
since  cuprous  copper  must  be  present,  even  if  the  iron  is  suspended  in  the 
solution  out  of  contact  with  the  pyrite. 

45  See  footnote,  page  446. 

46  Based  on  calculations  made  by  Dr.  E.  Posnjak  of  this  laboratory;  work 
unpublished. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  453 


the  reduction  of  cupric  copper  by  ferrous  iron  would  go  on  un- 
hindered. 

If  pyrite  is  added  to  a solution  containing  all  these  four  sub- 
stances, the  system  is  considerably  complicated  by  the  fact  that 
three  out  of  four  of  the  substances — all  except  the  ferrous  iron — 
react  with  pyrite.  The  action  of  cuprous  copper  on  pyrite  is 
similar  to  that  of  cupric  copper,  in  so  far  as  the  product  is  con- 
cerned, but  the  rate  of  reaction  of  the  latter  is  so  much  slower 
that  we  may  neglect  it  for  our  present  purposes.  Ferric  iron,  on 
the  other  hand,  has  quite  a different  action;  it  not  only  oxidizes 
the  pyrite  to  ferrous  salt  and  sulphuric  acid,  but  it  oxidizes  the 
enrichment  products,  dissolving  them  also.  The  ferric  iron  acts 
so  much  more  readily  on  the  sulphides  of  copper  than  on  pyrite 
that  in  the  case  of  a partially  enriched  pyrite  the  action  would  be 
entirely  confined  to  the  former.  Thus  the  chemical  action  of  the 
ferric  iron  is  to  dissolve  what  the  cuprous  copper  precipitates. 
If,  now,  the  ferric  iron  could  be  removed  from  solution,  the 
process  of  alteration  of  the  pyrite  would  undoubtedly  be  greatly 
accelerated. 

In  this  connection,  we  repeated  an  experiment  carried  out  by 
Spencer,47  who  added  calcite  to  a system  consisting  of  pyrite, 
cupric  and  ferrous  sulphates.  He  found  that  the  pyrite  became 
coated  with  a sulphide  of  copper  in  a comparatively  short  time. 
Our  results  confirm  Spencer’s  in  so  far  as  the  film  on  the  pyrite 
had  all  the  appearance  of  copper  sulphide,48  though  it  was  too  thin 
for  microscopic  examination. 

Too  little  is  yet  known  of  the  detailed  chemical  processes  of 
natural  enrichment  for  us  to  make  any  complete  application  of 
the  foregoing  facts  to  the  phenomena  of  nature;  we  can  only 
point  out  what  will  happen  in  certain  contingencies.  In  the  first 
place,  there  can  be  no  question  that  pyrite  and  other  ferriferous 
sulphides,  when  subjected  to  the  action  of  a copper  sulphate  solu- 
tion, will  give  a complex  system  containing,  besides  an  enriched 

47  Spencer,  loc.  cit.,  p.  649. 

48  Spencer,  loc.  cit.  Metallic  copper  also  was  observed.  Likewise,  it  was 
found  that  calcium  carbonate  does  precipitate  metallic  copper  from  a solution 
of  cupric  and  ferrous  sulphates. 


454 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


sulphide,  a solution  of  cupric,  ferrous,  cuprous,  and  ferric  sul- 
phates. In  the  second  place,  if  the  ferric  iron  is  removed  contem- 
poraneously with  the  enrichment  process,  the  latter  should  be 
greatly  accelerated. 

6.  ALTERATION  OF  PYRITE  TO  THE  COPPER-IRON  SULPHIDES. 

The  geological  evidence  seems  clear  that  pyrite  can  alter  to 
chalcopyrite  and  subsequently  to  bornite.  Up  to  the  present,  how- 
ever, we  have  not  been  able  to  induce  pyrite  to  alter  to  these  sul- 
phides when  cupric  sulphate  is  used  as  the  enriching  solution.  As 
a mattter  of  fact,  we  do  not  see  how  they  can  form  under  our 
conditions,  for  in  the  work  on  chalcopyrite  and  bornite  it  will  be 
shown  that  these  sulphides  are  attacked  by  cupric  sulphate  much 
more  rapidly  than  pyrite  at  all  temperatures.  It  is  rather  difficult 
to  see,  therefore,  how  these  sulphides  can  form  from  pyrite  when 
cupric  sulphate  is  the  enriching  solution.  This  must  especially 
be  the  case  with  bornite,  which,  as  will  appear  later  in  this  paper, 
is  very  sensitive  to  cupric  sulphate'  and  also  to  sulphuric  acid,  the 
latter  being  formed  when  pyrite  is.  attacked  by  cupric  sulphate. 

7.  THE  REACTION  BETWEEN  PYRRHOTITE  (FeSx)  AND 
CUPRIC  SULPHATE. 

A.  At  200 0 

Inasmuch  as  pyrrhotite  forms  one  of  the  primary  constituents 
of  a number  of  ore  bodies  which  give  evidence  of  having  under- 
gone secondary  enrichment,  it  was  thought  desirable  to  study  the 
reaction  between  pyrrhotite  and  solutions  of  cupric  sulphate. 
For  this  purpose  both  natural  and  synthetic  pyrrhotites  were  used. 
A number  of  preliminary  experiments  were  first  carried  out  in 
order  to  obtain  a clue  as  to  the  best  method  for  studying  the 
problem. 

In  these  preliminary  experiments  it  was  found  that  when  one 
gram  of  pyrrhotite  ground  to  pass  a‘  100-mesh  sieve  is  exposed 
to  the  action  of  50  c.c.  of  a 5 per  cent,  copper  sulphate  solution 
for  3 days  at  200°,  a very  energetic  reaction  takes  place  in  which 
all  of  the  copper  is  deposited.  In  addition  to  the  sulphide  en- 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  455 


richment  product,  hematite,  metallic  copper,  cuprite,  and  pyrite 
(or  marcasite)  were  formed.  Ferrous  sulphate,  hydrogen  sul- 
phide, and  sulphuric  acid  were  found  in  solution.  Pyrrhotite 
therefore  resembles  pyrite  in  behavior,  but  its  reaction  is  even 
more  complicated.  The  hematite,  metallic  copper,  and  cuprite, 
in  both  reactions  are  derived  in  the  same  way.  At  the  very  base 
of  the  residue  in  the  reaction  tube,  a few  grains  of  material  were 
noticed,  the  yellowish  color  of  which  bore  a striking  resemblance 
to  chalcopyrite.  Another  experiment  gave  a similar  result.  The 
pyrrhotite  and  its  alteration  products  were  so  caked  in  the  bottom 
of  the  tube  that  the  circulation  of  the  solution  through  the  mass 
must  have  been  greatly  impeded.  It  seemed  to  us  that  the  yel- 
low substance  might  have  been  the  first  product  to  form ; that  in 
its  formation  the  copper  in  the  immediate  vicinity  might  have 
been  entirely  removed  from  solution;  and  that  thereafter  the 
product  might  have  been  saved  from  further  change  by  the  fact 
that  additional  copper  could  reach  it  only  with  great  difficulty, 
owing  to  the  impeded  circulation  of  the  solution.  Acting  in  line 
with  this  suggestion,  in  the  next  experiment  we  increased  the 
surface  of  the  reacting  pyrrhotite,  and  to  make  the  action  more 
uniform  the  particles  were  separated  by  quartz.  Three  grams  of 
pyrrhotite  were  mixed  with  three  times  their  volume  of  pure 
quartz  and  exposed  at  200°  to  the  action  of  a 5 per  cent,  solution 
of  C11SO4 . 5H20.49  Under  these  conditions,  apparently  only  the 
yellow  product  was  obtained.  The  conditions  of  the  experiment 
were  thus  greatly  simplified,  and  it  now  remained  to  be  proved 
that  the  resulting  sulphide  was  chalcopyrite,  for  our  previous 
experience  with  pyrite  had  taught  us  not  to  trust  too  much  to 
surface  color  evidence.50 

Experiments  were  now  carried  out  on  synthetic  and  natural 
pyrrhotite  of  the  following  compositions : 


Pyrrhotite  Used. 

Fe. 

S. 

Cu. 

Synthetic  (1) 

38.12 

“ (2) 

61  17 

Auburn,  Maine 

38.92 

O.03 

49  See  footnote  12. 

50  See  page  439. 


456 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


The  synthetic  pyrrhotites  were  made  according  to  a method  de- 
veloped in  this  laboratory.51  Microscopical  examination  showed 
that  the  natural  pyrrhotite  contained  a very  minute  amount  of 
chalcopyrite.  Below  are  tabulated  the  initial  conditions  of  the 
experiments  at  200°. 

TABLE  XVI. 

Pyrrhotite  and  Cupric  Sulphate. 


Initial  Conditions  at  200°. 


Exp. 

Material. 

Weight,  g. 

Solution. 

Quantity. 

Copper 
Initial,  g. 

33 

Synthetic  (i) 

3.0000 

50  c.c.  5%  CUSO4.5H2O 

O.6363 

34 

“ 

“ 

“ 

“ 

35 

ii  <<  it 

4 4 

36 

(2) 

“ 

€t  44  44 

44 

37 

Auburn,  Maine 

<<  ll  n 

44 

In  all  of  the  experiments,  the  pyrrhotite  was  mixed  with  three 
times  its  volume  of  pure  quartz  sized  between  60  and  80  mesh. 
The  tubes  containing  the  pyrrhotite  and  solution  were  heated  at 
2000  for  a period  of  three  days.  At  the  end  of  this  time,  all  the 
copper  originally  present  in  solution  was  used  up  in  altering  the 
pyrrhotite. 

Analysis  of  Enrichment  Products  Replacing  the  Pyrrhotite. — 
The  residues  obtained  in  these  experiments  were  washed,  dried 
over  sulphuric  acid  in  a vacuum  desiccator,  and  analyzed  in  the 
following  manner : The  pyrrhotite  was  separated  from  the  quartz 
by  the  use  of  a 100-mesh  sieve,  the  pyrrhotite  passing  through 
and  most  of  the  quartz  remaining  on  the  sieve.  The  sulphides 
were  then  digested  with  1-10  sulphuric  acid  at  ioo°  in  an  atmos- 
phere of  H2S  in  order  to  remove  the  unattacked  pyrrhotite.  The 
wet  sulphides  were  then  ground  in  a mortar,  taking  care  to  simply 
break  up  the  grains  rather  than  to  grind  them  finely  by  any  rub- 
bing motion  of  the  pestle,  as  very  finely  divided  chalcopyrite  was 
found  to  be  attacked  by  the  acid.  After  regrinding,  the  sulphides 
were  again  digested  with  the  dilute  sulphuric  acid  in  an  atmos- 

51  Allen,  Crenshaw  and  Johnston,  Am.  J.  Sci.  (4),  33,  194  (1912). 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


457 


phere  of  H2S.  The  residue  was  then  dried  and  analzed.  Below 
are  tabulated  the  results  of  these  analyses ; the  values  given  have 
been  corrected  for  silica,  which  was  always  present  in  the  purified 
residues : 

TABLE  XVII. 

Pyrrhotite  and  Cupric  Sulphate. 


Analyses  of  Residues. 

Exp. 

Per  Cent, 
of  Cu. 

Per  Cent, 
of  Fe. 

Per  Cent, 
of  S. 

Summation. 

33 

32.39 

35-20 

33-50 

30.55 

29.50 

30.88 

28.OO 

36.44 

99.71 

34 

34a 

30.21 

33-20 

32.69 

36.07 

99.78 

36 

37 

30.12 

32.95 

Calculated  for  CuFeS2 

34.63 

30.42 

34-94 

The  chemical  analysis  was  supplemented  by  a microscopic  ex- 
amination. The  latter  revealed,  in  addition  to  chalcopyrite,  the 
presence  of  pyrite  and  pyrrhotite  in  all  of  the  purified  residues, 
thus  accounting  in  a large  measure  (except  in  experiment  34) 
for  the  discrepancy  between  the  calculated  values  for  chalcopyrite 
and  those  obtained  by  analysis  of  the  residues.  The  residues  from 
experiments  33  and  35  were  examined  by  Professor  Graton.  A 
portion  of  each  was  imbedded  in  sealing  wax,  polished,  and  ex- 
amined under  the  metallographic  microscope.  The  presence  of 
shells  of  chalcopyrite  was  noted  in  the  residue.  In  some  cases 
these  shells  contained  a core  of  pyrrhotite  but  in  most  cases  were 
hollow,  thus  indicating  that  the  pyrrhotite  had  been  removed  by 
the  treatment  with  acid.  We  found  that  the  shells  varied  in 
thickness  from  0.002  to  0.005  mm-  The  residue  obtained  in 
experiment  34  was  examined  before  treatment  with  acid,  and,  in 
addition  to  chalcopyrite  and  pyrrhotite,  bornite  also  was  found. 
This  residue  was  again  examined  after  purification  with  acid, 
and  it  was  found  that  the  bornite  had  altered  to  sulphides  of 
copper.  Later  on,  we  shall  show  that  bornite  is  easily  attacked 
by  sulphuric  acid  and  is  altered  by  it  to  the  sulphides  of  copper, 
thus  accounting  for  the  absence  of  bornite  when  the  residue  was 


458 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


examined  after  treatment  with  acid.  The  presence  of  these  sul- 
phides of  copper  also  accounts  for  the  high  percentage  of  copper 
found  in  34.  The  copper  sulphides  were  later  on  removed  with 
dilute  potassium  cyanide  and  the  residue  again  analyzed.  This 
analysis  is  shown  in  34a. 

Discussion. — The  microscopic  evidence,  together  with  the 
chemical  analysis,  show  that  chalcopyrite  is  the  end  product  when 
pyrrhotite  reacts  with  cupric  sulphate  at  200°,  if  the  surface  of 
the  pyrrhotite  and  the  amount  of  copper  in  solution  are  properly 
proportioned.  If  they  are  not  properly  proportioned,  the  chal- 
copyrite which  is  first  formed  envelops  the  pyrrhotite  to  such  an 
extent  as  to  prevent  ready  access  of  the  solution  to  the  pyrrhotite. 
When  this  happens,  the  cupric  sulphate  attacks  the  chalcopyrite, 
forming  sulphides  of  copper.  If  a large  excess  of  copper  is  pres- 
ent, hematite,  cuprite,  and  metallic  copper  are  formed.52 

In  view  of  the  fact  that  pyrrhotite  varies  in  chemical  compo- 
sition, it  is  obviously  impossible  to  establish  an  equation  which 
will  adequately  represent  the  reaction  involved  when  pyrrhotite 
and  cupric  sulphate  react  to  form  chalcopyrite,  consequently  no 
effort  was  made  to  analyze  quantitatively  the  solutions  obtained  in 
these  experiments.  Ferrous  sulphate  and  sulphuric  acid  were 
present  in  solution,  but  when  one  remembers  that  sulphuric  acid 
attacks  pyrrhotite  with  great  readiness,  it  becomes  obvious  that  the 
acid  found  at  the  end  of  an  experiment  is  not  the  total  acid  which 
is  formed  during  the  experiment ; the  acid  which  is  present  owes 
its  existence  to  the  fact  that  pyrrhotite  is  protected  from  attack  by 
the  chalcopyrite  which  envelops  it.  It  is  also  obvious  that  the 
ferrous  iron  in  solution  is  derived  partly  from  the  reaction  be- 
tween pyrrhotite  and  cupric  sulphate  and  partly  from  the  action 
of  the  acid  on  the  pyrrhotite ; the  hydrogen  sulphide  formed  in 
this  latter  action  can  not  of  course  persist  in  the  presence  of 
cupric  sulphate,  but  after  all  of  the  copper  has  been  precipitated 
considerable  of  the  gas  remains.  Subsequent  experiments  have 
led  us  to  believe  that  this  hydrogen  sulphide  plays  an  important 
52  See  page  446. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


459 


part  in  the  altering  of  pyrrhotite  into  chalcopyrite  under  these 
conditions.53 

It  is  evident  that  under  the  proper  conditions  pyrrhotite  and 
cupric  sulphate  react  very  readily  at  200°  to  form  chalcopyrite. 
In  our  experimental  work  on  the  reaction  between  pyrite  and 
cupric  sulphate,  we  did  not  succeed  in  obtaining  chalcopyrite  as 
an  alteration  product  on  pyrite,  and  we  doubted  very  much  that 
such  an  alteration  could  take  place  in  the  presence  of  cupric  sul- 
phate, owing  to  the  fact  that  chalcopyrite  reacts  more  readily 
with  cupric  sulphate  than  pyrite  does.  In  a subsequent  paper  on 
the  role  of  hydrogen  sulphide  we  hope  to  suggest  an  explanation 
for  the  difference  in  behavior  of  pyrite  and  pyrrhotite  towards 
cupric  sulphate.54 

B.  Pyrrhotite  and  Cupric  Sulphate  at  ioo°. 

These  experiments  were  carried  out  under  the  same  conditions 
as  those  at  200°,  namely,  the  pyrrhotite  was  mixed  with  three 
times  its  volume  of  pure  quartz  and  exposed,  in  evacuated  Jena 
glass  tubes,  to  the  action  of  the  cupric  sulphate  solution.  Below 
are  tabulated  the  initial  conditions  of  the  experiments  and  the 
analyses  of  the  resulting  solutions. 


TABLE  XVIII. 

Pyrrhotite  and  Cupric  Sulphate. 
Temperature,  ioo°. 


Initial  Conditions. 

Analyses  of  Solution. 

Exp. 

Dura- 

tion 

Weight, 

g. 

Copper,  g 

Days. 

Material. 

Solution. 

Initial. 

9 

Final. 

De- 

posited. 

38 

4586 

2.000 

Orange  Co.,  Vt.f 
125-200  mesh. 

25  c.c.  of  a 2 % 
CUS04.5H20. 

0.1272 

None 

O.I272 

39 

45 

1. 000 

Orange  Co.,  Vt., 
125-200  mesh. 

25  c.c.  of  a 2% 
CuSO-j.stLO. 

0.1272 

O.I272 

53  Work  in  hand. 

54  Work  in  hand. 

65  It  is  quite  possible  that  complete  deposition  of  the  copper  did  not  require 
this  long  period  of  time. 


460 


E.  G.  Z1ES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


This  natural  pyrrhotite  analyzed  as  follows:  Fe  = 6i.62  per 
cent.;  8 = 38.24  per  cent.;  Cu  = o.04  per  cent.  The  micro- 
scope revealed  a minute  amount  of  chalcopyrite. 

In  these  experiments  the  concentration  and  amount  of  the  solu- 
tion were  the  same,  but  the  quantity  of  sized  pyrrhotite  was 
varied.  In  experiment  38,  2 grams  of  pyrrhotite  were  used,  and 
in  this  case  the  color  of  the  chalcopyrite  was  very  pronounced;  a 
very  small  amount  of  some  oxide  of  iron  was  present;  no  hydro- 
gen sulphide  could  be  detected.  This  gas  was  tested  for  by  pass- 
ing hydrogen  through  the  solution  in  the  reaction  tube,  which  in 
turn  was  connected  with  an  absorption  apparatus  containing  cad- 
mium acetate.  The  solution  in  the  tube  contained  only  ferrous 
sulphate  and  a very  little  sulphuric  acid;  the  oxide  of  iron  re- 
ferred to  above  was  as  usual  derived  from  the  hydrolysis  of  the 
ferric  iron  formed  in  the  reaction  between  ferrous  sulphate  and 
cupric  sulphate,  which  also  takes  place  at  ioo°,  especially  when 
little  or  no  acid  is  present. 

A few  grains  of  the  deposit  were  examined  microscopically. 
It  was  shown  that  the  copper  was  deposited  as  chalcopyrite, 
which  replaced  the  outer  portion  of  the  grains  of  pyrrhotite. 

In  experiment  39,  where  only  one  gram  of  pyrrhotite  was  used, 
all  other  conditions  being  the  same,  the  residue  in  the  tube  pre- 
sented a somewhat  different  appearance;  instead  of  the  usual 
chalcopyrite  color,  the  bluish  color  often  seen  on  a badly  tarnished 
chalcopyrite  was  noted.  All  of  the  copper  originally  present  in 
solution  had  been  deposited.  Ferrous  sulphate,  hydrogen  sul- 
phide, and  9 milligrams  of  sulphuric  acid  were  present  in  solu- 
tion. The  solid  product  was  treated  with  a 1 per  cent,  solution 
of  KCN  in  the  cold,  which  completely  removed  the  tarnish  and 
exposed  the  characteristic  color  of  chalcopyrite,  the  presence  of 
which  was  further  proved  by  a mineralographic  examination. 

At  ioo°  we  were  thus  able  to  show  that  pyrrhotite  reacts  with 
a solution  of  cupric  sulphate  to  form  chalcopyrite,  and  that  this 
chalcopyrite  is  further  attacked,  forming  sulphides  of  copper,  if 
the  reacting  surface  of  the  pyrrhotite  is  not  large  enough  to 
rapidly  use  up  all  the  copper  in  solution. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  461 
C.  Experiments  at  40° . 

Finally,  experiments  with  pyrrhotite  and  cupric  sulphate  were 
carried  out  at  40°.  Two  natural  pyrrhotites  differing  in  chem- 
ical composition  were  used;  these  were  carefully  sized  in  the 
usual  manner  between  125  and  200  mesh.56  These  pyrrhotites 
analyzed  as  follows : 


T Per  Cent. 

Locality.  t of  Fe_ 

Per  Cent, 
of  S. 

Per  Cent, 
of  Cu. 

Per  Cent, 
of  Residue. 

Copper  Mt.,  Alaska 61.10 

Orange  Co.,  Vermont 61.62 

38.68 

38.24 

00.06 

OO.O4 

00.06 

These  experiments  were  carried  out  in  the  shaking  machine 
described  on  page  415.  If  it  is  desired  to  compare  the  relative 
reactivity  of  two  pyrrhotites,  differing  in  chemical  composition, 
towards  cupric  sulphate,  the  bottles  and  their  contents  should  be 
shaken  rather  gently  because  pyrrhotite  is  a soft  mineral  and 
easily  broken  by  attrition.  The  necessity  of  doing  this  will  be 
especially  evident  when  we  consider  that  the  duration  of  these 
experiments  was  two  months.  Below  are  tabulated  the  initial 
conditions  under  which  the  experiments  were  carried  out  and 
also  the  analyses  of  the  resulting  solutions : 


TABLE  XIX. 

Pyrrhotite  and  Cupric  Sulphate  at  Ordinary  Temperatures. 


Initial  Conditions. 

Analyses  of  Solutions. 

Exp. 

Dura- 

tion, 

Months. 

Weight, 

Material. 

Solution. 

Copper,  g 

Initial.  Final. 

Depos- 

ited. 

40 

2 

4-759 

Orange  Co., 
125-200  mesh. 

l£%  C11SO4.5H2O 
in  400  c.c. 

I.2765  I.2285 

0.0480 

41 

LT 

4-759 

Copper  Mx, 
125-200  mesh. 

i\%  CUSO4.5H2O 
in  400  c.c. 

I.2765  1.2003 

0.0762 

In  both  experiments,  the  pyrrhotite  changed  color  after  the. first 
12  hours  at  40°,  assuming  a yellowish  cast  which  became  a pro- 
56  See  page  410. 


462 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


nounced  chalcopyrite  yellow  in  about  seven  days.  The  yellow 
color  persisted  in  the  case  of  the  pyrrhotite  from  Orange  County 
for  about  seven  days  more,  but  in  both  experiments  the  yellow 
color  slowly  gave  way  to  a bronze-like  color  which  persisted  in  40 
until  the  end  of  the  experiment.  Subsequent  examination  with 
the  microscope  showed  that  this  bronze-like  color  was  due  to 
iridescence  caused  by  a very  thin  film  of  sulphide  which  had 
formed  on  the  chalcopyrite.  The  color  in  the  case  of  the  pyr- 
rhotite from  Copper  Mt.  was  not  bronze-like,  but  decidedly  blu- 
ish, and  also  showed  iridescence  when  examined  under  the 
microscope. 

The  conditions  of  the  experiments  were  made  as  nearly  com- 
parable as  possible,  so  that  this  rather  striking  difference  in 
amount  of  copper  deposited  is  significant  and  can  be  attributed 
either  to  the  difference  in  chemical  composition  or  to  some  differ- 
ence in  physical  structure,  meaning  by  the  latter  that  the  more 
reactive  specimen  may  have  been  more  fissured  and  thus  pre- 
sented a greater  surface  to  the  solution.  Additional  work  must 
be  done,  however,  in  order  to  prove  this  point,  and  it  is  especially 
essential  to  secure  a large  number  of  pure  natural  pyrrhotites  dif- 
fering in  chemical  composition  and  in  physical  structure,  and 
likewise  to  secure  a number  of  synthetic  pyrrhotites  differing  in 
chemical  composition.  The  securing  of  the  natural  pyrrhotites 
is  in  itself  a rather  difficult  problem. 

The  residues  obtained  in  these  experiments  were  compressed 
into  tablets  in  the  manner  previously  described,  polished,  and  ex- 
amined under  the  microscope.  The  films  formed  on  the  pyrrho- 
tite were,  however,  too  thin  to  be  identified  by  this  method.  Both 
residues  were  then  treated  with  a dilute  solution  of  KCN  '( J4  per 
cent.).  In  a few  minutes  the  tarnish  disappeared,  giving  way  to 
the  characteristic  color  of  chalcopyrite,  a uniform  yellow,  and  not 
due  to  iridescence.  The  color  evidence  seems  strong  enough  to 
warrant  our  making  the  statement  that  at  ordinary  temperatures,, 
as  well  as  at  higher  temperatures,  chalcopyrite  is  the  first  enrich- 
ment sulphide  formed  when  pyrrhotite  reacts  with  a solution  of 
cupric  sulphate. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  463 


8.  THE  REACTION  BETWEEN  CHALCOPYRITE  (CuFeS2)  AND 
CUPRIC  SULPHATE. 

It  is  known  that  chalcopyrite  in  nature  alters  into  cuprous  sul- 
phide and  also  into  cupric  sulphide.  Efforts  were  made  to  bring 
about  these  alterations  and  to  study  the  reactions  involved. 

The  experimental  work  on  chalcopyrite  was  done  in  much  the 
same  manner  as  that  indicated  under  pyrite.  This  sulphide,  how- 
ever, is  reactive  enough  towards  cupric  sulphate  solutions  at  ordi- 
nary temperatures  to  make  experimental  work  worth  while.  In 
all  of  the  experiments  above  40 °,  the  sulphide  was  mixed  with 
three  times  its  volume  of  pure  quartz  and  exposed  to  the  action 
of  the  solutions  in  the  usual  manner.  When  the  experiment 
extended  over  a longer  period  of  time  than  three  days,  silica 
tubes  were  used  instead  of  Jena  glass  tubes. 

A.  Chalcopyrite  and  Cupric  Sulphate  at  200° . 

The  behavior  of  chalcopyrite  with  cupric  sulphate  resembles 
that  of  pyrite  in  that  the  secondary  products,  hematite,  cuprite 
and  metallic  copper  are  likely  to  form;  in  fact,  chalcopyrite  can 
not  be  converted  entirely  into  chalcocite  in  this  way  at  200 0 with- 
out obtaining  these  secondary  products.  In  such  a case  the  mix- 
ture is  of  course  too  complicated  for  analysis.  We  have  shown 
that  these  secondary  products,  when  obtained  in  the  alteration  of 
pyrite,  will  eventually  redissolve  owing  to  the  action  of  the  sul- 
phuric acid  which  the  oxidation  of  the  sulphur  in  pyrite  yields. 
Chalcopyrite  yields  much  less  acid,  hence  it  is  necessary  to  have 
acid  present  as  one  of  the  initial  constituents  of  the  solution  when 
it  is  desired  to  convert  chalcopyrite  at  the  higher  temperatures 
completely  into  chalcocite.  Before  carrying  out  quantitative  ex- 
periments with  chalcopyrite  and  cupric  sulphate  together  with  sul- 
phuric acid,  it  was  first  necessary  to  learn  if  sulphuric  acid  readily 
attacks  chalcopyrite  at  200°. 

(a).  Action  of  Sulphuric  Acid  on  Chalcopyrite . 

The  experiments  with  chalcopyrite  which  we  are  about  to  de- 


464 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


scribe  are  of  a qualitative  nature  only.  The  chalcopyrite  used 
was  obtained  from  Evora,  Portugal,  and  analyzed  as  follows  :57 


Evora,  Portugal  Cu  = 34.36;  Fe  = 30.61;  S = 35.01 

Calculated  for  CuFeS2  Cu  = 34.63;  Fe  = 30.42;  S = 34.94 


Experiment  42.  Action  of  Acid  at  200°. — One  gram  of  this 
chalcopyrite  was  sized  between  125  and  200  mesh  and  treated 
with  50  c.c.  of  2 per  cent,  sulphuric  acid  at  200°  for  a period  of 
two  days.  Ferrous  sulphate,  and  hydrogen  sulphide  were  found 
in  solution.  While  it  is  true  that  these  experiments  were  of  a 
qualitative  nature  only,  yet  we  are  justified  in  making  the  state- 
ment that  the  amounts  of  ferrous  sulphate  and  hydrogen  sulphide 
found  were  far  greater  than  those  found  when  pyrite  was  simi- 
larly treated.58  In  addition  to  noting  the  presence  of  these  sol- 
uble products,  the  presence  of  some  sulphide  of  copper  was  ob- 
served as  a coating  on  the  chalcopyrite.  In  another  experiment, 
carried  out  under  the  same  conditions  and  extending  over  a period 
of  ten  days,  the  presence  of  crystal  aggregates,  together  with  the 
other  products  just  mentioned,  was  observed.  These  were  ex- 
amined microscopically  and  shown  to  be  marcasite.59 

Action  i of  Acid  at  ioo°. — Chalcopyrite  sized  between  125  and 
200  mesh  was  exposed  to  the  action  of  sulphuric  acid  also  at  ioo°. 
The  duration  of  the  experiment  was  2 weeks.  If  hydrogen  sul- 
phide was  formed  in  this  case,  the  amount  was  less  than  could  be 
detected  by  the  method  of  analysis  detailed  under  pyrite.  This 
method  we  know  to  be  very  accurate. 

( b ) Alteration  of  Chalcopyrite  t\o  Chalcocite. 

As  a result  of  our  experiments  on  chalcopyrite  and  sulphuric 
acid  at  200°,  we  could  see  no  reason  for  believing  that  the  acid 
could  in  any  way  seriously  affect  the  formation  of  cuprous  sul- 

57  Analysis  by  Dr.  J.  L.  Crenshaw. 

58  Pyrite  when  treated  under  similar  conditions  yielded  0.6  milligram  of 
hydrogen  sulphide,  whereas  chalcopyrite  yielded  about  39  milligrams. 

59  This  is  the  form  of  FeS2  obtained  at  ordinary  temperatures  when  the 
acid  is  of  the  concentration  indicated.  See  Allen,  Crenshaw  and  Merwin, 
Am.  J.  Sci.  (4),  38,  393,  1914. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4^5 


phide,  therefore  in  the  experiments  which  immediately  follow  we 
used  sulphuric  acid  in  order  to  prevent  the  formation  of  the  sec- 
ondary products  mentioned  on  page  446. 


TABLE  XX. 

Chalcopyrite  and  Cupric  Sulphate. 

Initial  Conditions:  0.4000  g.  chalcopyrite  from  Evora  (125-200  mesh)  ; solu- 
tion, 25  c.c.  5 per  cent.  CuS04-5H20  and  about  2^4%  H2S04; 
temperature,  2000.60 


Analyses  of  the  Solutions. 


Exp. 

Duration, 

Days. 

Copper,  g. 

Acid,  g. 

Total  Iron 
(Ferrous), 
gr- 

Initial. 

Final. 

Deposited. 

Initial. 

Final. 

Formed. 

43 

8 

0.3197 

O.0248 

O.2949 

0.6063 

O.9410 

0-3347 

O.I2I7 

44  1 

8 

0.3197 

0.0205 

O.2992 

0.6063 

O.9502 

0-3439 

O.I222 

All  of  the  copper  lost  by  the  solution  was  precipitated  on  the 
chalcopyrite  as  sulphide,  and  exhibited  the  characteristic  prop- 
erties of  an  enrichment  product  in  that  it  adhered  firmly  and  re- 
placed the  chalcopyrite. 

Molecular  Ratios. — On  the  basis  of  the  above  analyses,  the 
following  molecular  ratios  were  determined : 


Exp. 

Cu 

H2SO4 

Cu 

FV 

Fe 

h2so4* 

43 

2.13 

i-57 

1.36 

44 

2.15 

1.60 

1.34 

The  residues  were  compressed,  polished,  and  examined  micro- 
scopically. In  experiment  43  this  examination  proved  the  pres- 
ence of  cuprous -sulphide  together  with  a few  per  cent,  of  cupric 
sulphide;  likewise  it  was  shown  that  a few  grains  of  unaltered 
chalcopyrite  were  still  present.  In  44,  it  was  shown  that  in  so 
far  as  the  microscope  could  determine,  all  of  the  chalcopyrite  had 
altered  to  cuprous  sulphide.  This  evidence  taken  together  with 

60  Similar  experiments  were  made  with  no  acid  present  initially,  but  the 
complications  mentioned  on  page  464  forced  us  to  discontinue  experimentation 
along  this  line. 


466  E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 

the  ratios  points  to  the  following  equation  as  representing  the 
reaction  between  chalcopyrite  and  cupric  sulphate  under  such 
conditions : 

5CuFeS2+nCuS04+8H20=8Cu2S+5FeS04+8H2S04  (7) 

The  ratios  demanded  by  this  equation  are  the  following : 


Cu 

H2SO4 

Cu 

Fe  ‘ 

Fe  ’ 

H2SO/ 

2.2 

1.6 

1-375 

The  agreement  between  these  ratios  and  those  obtained  in  experi- 
ment 44  are  as  close  as  the  experimental  errors  involved  will 
permit.  The  microscopical  examination  of  the  residue  in  43 
showed  that  cupric  sulphide  was  present  along  with  the  cuprous 
sulphide. 

The  enrichment  products  obtained  in  both  experiments  were 
dissolved  in  a 2 per  cent,  potassium  cyanide  solution  and  ana- 
lyzed in  the  usual  manner.  In  view  of  the  very  small  amount  of 
chalcopyrite  left  in  the  residue  the  error  introduced  into  the  anal- 
ysis, due  to  the  slight  solubility  of  the  chalcopyrite  in  the  2 per 
cent,  solution  of  potassium  cyanide,  is  negligible. 


TABLE  XXI. 

Chalcopyrite  and  Cupric  Sulphate. 

Analyses  of  the  Enrichment  Products.  Temperature,  200  . 


_ Cu  in  KCN, 

S in  KCN, 

Total, 

Per  Cent. 

Per  Cent. 

Per  Cent. 

Exp.  j g. 

g- 

g. 

of  Cu. 

of  CU2S. 

of  CuS. 

43  0.1460 

O.O39O 

O.1850 

78.93 

93  ± 1% 

7 

44  1 0.1471 

0.0375 

O.1846 

79.71 

99  “ 

I 

These  analyses  confirm  the  observations  made  microscopically, 
namely,  that  in  experiment  44  the  chalcopyrite  had  altered  to 
chalcocite,  and  that  in  43  both  cupric  and  cuprous  sulphides  were 
present. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4 6/ 


( C ) ALTERATIONS  OF  CHALCOPYRITE  TO  CUPRIC  AND  CUPROUS 
SULPHIDES  AT  200°. 

Efforts  were  made  to  find  conditions  under  which  chalcopyrite 
and  cupric  sulphate  will  react  to  form  cupric  sulphide  as  the  only 
enrichment  product.  In  this  we  were  not  successful,  but  we  have 
been  able  to  show  that  chalcopyrite  does  alter  into  cupric  sulphide 
with  subsequent  alteration  into  cuprous  sulphide;  the  amount  of 
cupric  sulphide  which  can  form  depending  on  the  conditions  of 
the  experiment. 

TABLE  XXII. 

Chalcopyrite  and  Cupric  Sulphate. 

Initial  Conditions : Evora  chalcopyrite  (ioo  mesh  and  finer)  ; solution,  50  c.c. 

5 per  cent.  CuS04-5H20;  temperature,  200°. 


Dura- 

tion, 

Days. 

Analyses  of  the  Solutions. 

Exp. 

Weight, 

g ■ 

Copper,  g. 

j 

Acid 

h2so4, 

g ■ 

Total  Iron 
(Ferrous), 
g • 

Initiai. 

Final. 

Deposited. 

45 

25 

1. 000 

O.6363 

O.OO08 

O.6335 

0.5085 

O.3026 

46 

3 

5.000 

O.6363 

O.OO44 

O.6319  1 

0.2782 

0.4304 

In  the  experiments  on  the  alteration  of  chalcopyrite  into  cu- 
prous sulphide,  it  was  found  necessary  to  have  sulphuric  acid 
present  as  one  of  the  initial  constituents  of  the  solution.  This 
was  not  found  necessary  when  the  conditions  shown  in  Table 
XXII.  obtain.  In  neither  experiment  was  hematite  present  at  the 
end  of  the  experiment.  The  tubes  were  left  in  the  furnace  at 
200°  until  the  solutions  had  become  colorless,  indicating  that  most 
of  the  copper  had  been  deposited. 

Increasing  the  amount  of  this  finely  divided  chalcopyrite  from 
one  to  five  grams  has  evidently  caused  marked  change  in  the 
length  of  time  required  for  the  solution  to  become  colorless. 

The  analyses  bring  out  two  other  interesting  points.  It  will  be 
noted  that  in  experiment  46,  five  times  as  much  chalcopyrite  was 
used  as  in  45  and  that  in  both  cases  quartz  was  added  to  make 
available,  as  nearly  as  possible,  the  total  surface  of  chalcopyrite, 
yet  in  46  much  less  acid  and  much  more  iron  were  present  in  solu- 


468 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


tion  at  the  end  of  the  experiment.  The  increase  in  the  amount 
of  iron  shows  that  a greater  amount  of  chalcopyrite  reacted  in  46 
than  in  45,  and  the  decrease  in  the  amount  of  acid  shows  that  less 
oxidation  of  the  sulphur  by  the  cupric  sulphate  took  place  in  46 
than  in  45.  This  fact  is  shown  somewhat  more  clearly  by  the 
molecular  ratios  derived  from  the  above  analyses : 


Exp. 

Cu 

H2SO4 

Cu 

Fe' 

Fe  ' 

H^SCV 

45 

1.85 

O.96 

1-93 

46 

I.29 

0-37 

3-50 

These  ratios  differ  greatly  not  only  from  one  another  but  also 
from  the  ratios  found  when  chalcopyrite  alters  into  cuprous  sul- 
phide. The  ratio  Cu/Fe  is  approaching  unity,  the  ratio  H2S04/Fe 
is  approaching  o,  and  the  ratio  Cu/H2S04  is  increasing  rapidly. 

The  residues  obtained  in  these  experiments  were  examined 
microscopically.61  In  addition  to  the  unaltered  chalcopyrite,  the 
presence  of  cupric  and  cuprous  sulphides  as  alteration  products 
on  the  chalcopyrite  was  established. 

All  the  evidence  brought  forward  leads  us  to  believe  that  chal- 
copyrite when  exposed  at  200°  to  the  action  of  cupric  sulphate 
will  alter  to  cupric  and  cuprous  sulphides,  and  that  the  greater  the 
percentage  of  cupric  sulphide  present  in  the  enrichment  product 
the  greater  will  be  the  amount  of  iron  obtained  when  a given 
amount  of  copper  as  sulphate  reacts  with  chalcopyrite  and  the 
smaller  will  be  the  amount  of  sulphuric  acid;  also,  the  greater 
the  surface  of  chalcopyrite  exposed  to  a given  amount  of  cupric 
sulphate,  the  greater  is  the  tendency  to  form  cupric  sulphide.62 

B.  The  Reaction  between  Chalcopyrite  and  Cupric  Sulphate 

at  40°. 

In  order  to  obtain  additional  evidence  along  this  line,  the  fol- 
lowing experiments  were  carried  out  in  the  shaking  machine  at 

61  Examined  by  Graton  and  Murdoch. 

62  See  page  442.  The  argument  used  in  the  case  of  pyrite  applies  here  also. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  469 


40°,  using  chalcopyrite  sized  between  125  and  200  mesh.63  The 
object  of  this  sizing  will  be  referred  to  later.64  The  chalcopyrite 
known  as  Japan  “y,”  analyzed  as  follows: 


Per  Cent,  of  Cu. 

Per  Cent,  of  Fe. 

Per  Cent,  of  S. 

Japan  “y”65 

34-63 

34-63 

30.70 

30.42 

Calculated  for  CuFeS2 

34-94 

TABLE  XXIII. 

Chalcopyrite  and  Cupric  Sulphate.  Temperature,  40°. 


Initial  Conditions. 

Analyses  of  Solutions. 

Exp. 

Dura- 

tion, 

Months. 

Weight, 

g- 

Copper,  g 

Total 
Iron,  g. 

Acid, 

Material. 

Solution. 

Initial. 

Final. 

Depos- 

ited. 

H2SO4, 

g- 

47 

2 

12.000 

Ugo,  Japan,66 
125-200  mesh 

200  C.C.  l\ % 

CUS04.5H20 

O.6359 

O.5740 

O.0619 

O.0437 

0.020 

48 

2 

4-253 

Japan  “y” 
125-200  mesh 

400  C.C.  Ij% 
CUS04.5H26 

I.2744 

1.2538 

0.0206 

O.OI36 

49 

2 

5.000 

Evora, 

125-200  mesh 

400 C.C.  l\% 
CUS04.5H20 

I.2718 

1. 2410 

0.0308 

O.OI92 

The  chalcopyrite  gradually  became  coated  with  a thin  iridescent 
film  whose  color  resembled  bornite67  but  which  the  analysis 
showed  to  be  sulphides  of  copper.68 

The  copper  and  iron  were  determined  in  aliquot  portions  of  the 
solution,  and  both  are  correct  within  0.5  milligram  of  substance. 
The  copper  was  determined  electrolytically  as  usual ; the  iron  was 
determined  gravimetrically  after  removing  the  copper  with  hy- 
drogen sulphide  under  the  proper  conditions.  The  gravimetric 
determination  was  checked  by  dissolving  the  oxide  of  iron  in 
potassium  bisulphate,  reducing  and  titrating  with  potassium  per- 
manganate. Sulphuric  acid  was  always  present  in  the  solutions, 
but  in  such  small  amounts  that  its  determination  in  the  presence 
of  copper  could  not  be  accurately  made  except  in  experiment  47, 

63  See  page  410. 

64  See  page  471. 

65  Analysis  by  Dr.  J.  L.  Crenshaw. 

66  Examined  microscopically  and  found  to  be  very  pure. 

67  See  also  Welsh  and  Stewart,  Econ.  Geol.,  7,  785-7,  1912. 

68  See  page  439. 


4;o 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


in  which  20  milligrams  of  H2S04  were  found;  this  determina- 
tion is  correct  to  within  1 milligram.69 

The  following  molecular  ratios  were  calculated  on  the  basis  of 
these  analyses : 


Exp. 

Cu 

Fe' 

Fe 

H2S04‘ 

Cu 

h2so4' 

47 

I.24  ± O.05 

0-3 

4.8 

48 

1.3  ±0.1 

49 

1.4  ± 0.06 

Analysis  of  Enrichment  Products. — The  enrichment  products 
formed  on  the  chalcopyrite  in  experiment  47  were  brought  into 
solution  with  1 per  cent.  KCN.  If  the  operation  is  done  rapidly 
and  the  chalcopyrite  sized  as  indicated,  the  error  introduced 
into  the  analysis  by  the  solubility  of  the  chalcopyrite  in  the  KCN 
is  small  and  can  easily  be  corrected  for  by  exposing  pure  chalco- 
pyrite to  the  action  of  the  1 per  cent,  solution  for  the  same  length 
of  time  as  that  which  elapsed  between  the  disappearance  of  the 
coating  and  completion  of  the  filtration;  the  same  amount  of 
sized  chalcopyrite  must  of  course  be  used  in  the  blank  as  was 
used  in  the  experiment.  The  correction  amounted  in  the  follow- 
ing analysis  to  0.4  milligram  of  copper,  and  the  same  amount  of 
sulphur,  quantities  obviously  negligible. 


Analysis  of  Enrichment  Products. 


1 

Exp. 

Cu  in  KCN. 
g. 

S in  KCN. 
g. 

Total, 

g. 

Per  Cent,  of 
Cu. 

Per  Cent,  of 
CU2S. 

Per  Cent,  of 
CuS  by 
Difference. 

47 

0.0871 

0.0386 

O.1258 

69.30 

21  ± 3 

79 

(a)  Influence  of  Sulphuric  Acid. 

In  the  case  of  pyrite,  we  found  that  sulphuric  acid  retarded  the 
reaction  between  pyrite  and  cupric  sulphate ; efforts  were  made  to 
learn  if  this  acid  would  also  retard  the  reaction  between  chalco- 
pyrite and  cupric  sulphate.  This  work  was  carried  out  at  40° 
and  is  tabulated  below. 

69  The  total  amount  of  the  solution  was  used  in  making  this  determination. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


4 71 


The  amount  of  copper  deposited  can  be  determined  with  great 
accuracy,  consequently  the  conclusion  is  justified  that  the  rela- 
tively large  difference  in  the  amounts  shown  in  the  table  is  due 
to  the  retarding  influence  of  the  2 per  cent,  sulphuric  acid. 


TABLE  XXIV. 

Influence  of  Sulphuric  Acid,  Chalcopyrite  and  Cupric  Sulphate. 


Initial  Conditions. 

Analyses  of  Solutions. 

Exp. 

Dura- 

tion, 

I 

Weight, 

** 

Copper,  g. 

Acid, 

Initial. 

Total 

Months. 

Material. 

Solution. 

Initial. 

Final. 

Depos- 

ited. 

Iron, 

g. 

47 

2 

12.000 

Ugo,  Japan, 
125-200  mesh 

200  C.C.  l\% 

C11SO4 . 5H2O 
No  acid. 

O.6359 

0.5740 

0.0619 

None 

0.0437 

50 

2 

12.000 

Ugo,  Japan, 
125-200  mesh 

200  C.C.  i|% 
C11SO4.5H2O 
land  2%H2S04 

0.6359 

0.5915 

O.0444 

2% 

O.O376 

( b ) Influence  of  Ferrous  Sulphate  at  400.70 
TABLE  XXV. 

Initial  Conditions. 


Exp. 

Material. 

Weight, 

g- 

Solution. 

Duration. 

47 

Ugo,  Japan,  125-200  mesh 

12.000 

200  C.C.  Ij%  C11SO4.5H2O.  . . . 

2 months 

5i 

n it  << 

“ 

200  C.C.  Ij%  C11SO4.5H2O 

and  1%  FeSC>4.7H20 

“ “ 

The  solution  in  both  experiments  became  cloudy  after  three 
days  owing  to  the  hydrolysis  of  the  ferric  iron  formed  by  the 
action  of  cupric  sulphate  on  the  ferrous  sulphate.  This  cloudi- 
ness gradually  increased,  reached  a maximum,  and  then  slowly 
disappeared.  During  this  time  a marked  difference  in  the  ap- 
pearance of  the  coatings  formed  on  the  chalcopyrite  was  noted. 
The  color  of  the  coatings  in  both  experiments  resembled  that  of  a 
tarnished  bornite;  previous  experiments  had  shown  that  this 
appearance  is  a kind  of  composite  effect,  inasmuch  as  the  films 
were  so  very  thin  that  iridescence  was  noted  when  the  substances 
70  See  also  A.  C.  Spencer,  Econ.  Geol.,  8,  625  (1913). 


472 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


were  examined  under  the  microscope.  In  experiment  51,  in 
which  ferrous  sulphate  was  used,  the  coating  at  the  end  of  five 
days  became  perceptibly  darker  than  the  coating  formed  in  47. 
The  difference  became  more  marked  and  reached  its  maximum 
about  the  same  time  that  the  maximum  cloudiness  of  the  solution 
was  noted.  When  the  cloudiness  of  the  solution  decreased,  the 
difference  in  appearance  of  the  coatings  decreased,  until  at  the 
end  of  the  experiment,  which  extended  over  a period  of  two 
months,  no  difference  in  appearance  was  noted,  an  observation 
borne  out  by  the  analysis  of  the  solutions. 


TABLE  XXVI. 
Analyses  of  the  Solutions. 


Exp. 

Copper  Initial,  g. 

Copper  Final. 

Copper  Deposited. 

47 

O.6359 

0.5740 

O.0619 

51 

O.6359 

O.5790 

O.0569 

As  a matter  of  fact,  at  the  end  of  two  months  less  copper  was 
deposited  when  ferrous  sulphate  was  used. 

The  coatings  were  dissolved  in  the  usual  manner  with  a 1 per 
cent,  solution  of  KCN  and  analyzed. 

TABLE  XXVII. 

Analysis  of  the  Enrichment  Products. 


Exp. 

Cu  in  KCN,  g. 

Cu  KCN,  g. 

Total,  g. 

Per  Cent. 

. of  Cu. 

Per  Cent, 
of  Cu2S. 

Per  Cent,  of 
CuS  by  Dif- 
ference. 

47 

Si 

N 1> 

r-  ^ 
00  t'- 
0 0 
d d 

0.0386 

O.0324 

O.1258 

O.IO71 

69.30 

69.74 

21  d=  3 

24  ± 3 

79 

76 

The  cupric  and  cuprous  sulphides  agree  within  the  limits  of  the 
experimental  error,  consequently  the  ferrous  sulphate  has  ex- 
erted no  appreciable  influence  in  changing  the  relative  amounts 
of  these  sulphides.  The  changes  observed  in  the  appearance  of 
the  coating  of  copper  sulphides  on  the  chalcopyrite  when  ferrous 
sulphate  is  present  as  an  initial  constituent  of  the  solution  can  be 
explained  on  the  basis  of  the  reaction  represented  by  equation 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  473 


(4).  The  cloudiness  of  the  solution  was  caused  by  the  precipita- 
tion of  the  hydrolyzed  ferric  iron.  The  more  extensive  altera- 
tion of  the  chalcopyrite  at  first  was  caused  by  the  reaction  be- 
tween chalcopyrite  and  cuprous  sulphate.  The  reaction  between 
chalcopyrite  and  cuprous  sulphate  continued  until  the  hydrolyzed 
ferric  iron  and  the  resulting  sulphuric  acid  were  in  equilibrium.  We 
have  seen  from  experiment  47  that  in  the  reaction  between  chalco- 
pyrite and  cupric  sulphate,  sulphuric  acid  is  also  formed,  and  this 
latter  acid  will  disturb  the  equilibrium  just  mentioned  and,  in  the 
course  of  time,  all  of  the  precipitated  ferric  iron  will  be  redis- 
solved. While  this  is  going  on  the  ferric  iron  will  carry  a corre- 
sponding amount  of  copper  back  into  solution.  Thus  at  the  end 
of  the  experiment,  the  influence  of  the  ferrous  sulphate  must  be 
reduced  to  a negligible  quantity.  The  experiment  showed  that 
such  was  the  case. 

C.  Discussion  of  Experimental  Work  with  Chalcopyrite. 

When  we  compare  the  molecular  ratios  obtained  in  our  experi- 
mental work  on  the  reaction  between  cupric  sulphate  and  chalco- 
pyrite, we  note  the  following  interesting  features : 


TABLE  XXVIII. 


Exp. 

Temp. 

Molecular  Ratios. 

Analyses  of  Enrichment 
Products. 

Cu 

IT 

H2S04 

Cu 

Per  Cent,  of 
Cu2S. 

Per  Cent,  of 
Cu3. 

Fe 

H2S04 

44 

200 

2.13 

I.60 

1-34 

99 

I 

43 

* ‘ 

2.15 

1-57 

1.36 

93 

7 

45 

* ‘ 

1.85 

0.96 

1-93 

49 

40 

1.4 

48 

* * 

1-3 

46 

200 

1.29 

0-37 

3-50 

47 

40 

1.24 

0.3 

__4l8 

21 

79 

The  analysis  of  the  enrichment  products  formed  in  44  and  43 
at  200°  shows  that  mostly  cuprous  sulphide  was  present,  whereas 
in  49  at  40°  mostly  cupric  sulphide  was  present.  Judging  from 
these  results,  the  formation  of  the  maximum  amount  of  cupric 
sulphide  is  accompanied  with  a minimum  of  sulphuric  acid.  The 


474 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


limits  which  the  ratios  Cu/Fe  and  H2S04/Fe  are  approaching 
indicate  that  no  acid  whatsoever  will  be  formed  when  chalcopyrite 
alters  only  into  cupric  sulphide.  The  simplest  equation  which 
expresses  these  facts  is  the  following : 

CuFeS2  + CuS04  = 2-CuS  + FeS04.  (8) 

The  cupric  sulphide  formed  in  this  equation  is  attacked  by  the 
cupric  sulphate  still  present  and  altered  into  cuprous  sulphide; 
this  action  increasing  as  the  chalcopyrite  becomes  coated  with  its 
alteration  products  which  render  it  less  accessible  to  the  solution. 

If  we  assume  that  all  of  the  iron  found  at  the  end  of  an  ex- 
periment was  due  to  the  reaction  represented  by  (8),  and  that  the 
acid  found  was  due  to  the  reaction  between  covellite  and  cupric 
sulphate  which  we  have  shown  to  take  place  not  only  at  elevated 
temperatures  but  also  at  ordinary  temperatures,71  namely, 

5CuS  -f-  3CuS04  4H20  = 4Cu2S  -f-  4H2S04,  ( i ) 

we  should  be  able  to  calculate  on  the  basis  of  these  equations  the 
amount  of  cupric  and  cuprous  sulphide  present  at  the  end  of  an 
experiment.  This  was  done  in  the  case  of  experiment  47,  page 
4 73,  and  the  values  found  compared  with  those  obtained  by  anal- 
ysis of  the  enrichment  product. 


CuS  Cu2S 

By  analysis 79  + 3 21+3 

Calculated  75  25 


The  agreement  is  as  close  as  can  be  expected  in  view  of  the  ex- 
perimental errors  and  difficulties  involved. 

9.  THE  REACTION  BETWEEN  CUPRIC  SULPHATE  AND  BORNITE 

(Cu5FeS4). 

Bornite  proved  to  be  the  most  difficult  and  at  the  same  time 
71  See  pages  491  and  496. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  475 


the  most  interesting  in  its  behavior  of  any  natural  sulphide  which 
we  studied.  Bornite  was  found  to  react  even  at  ordinary  tem- 
peratures rather  easily  with  sulphuric  acid  and  also  with  cupric 
sulphate.  It  is  likewise  attacked  by  dilute  solutions  of  potassium 
cyanide.  The  enrichment  products  formed  by  replacement  of 
the  other  natural  sulphides  studied,  could,  by  observing  the  proper 
precautions,  !be  removed  with  potassium  cyanide  without  affecting 
the  unchanged  sulphide  appreciably  and  the  resulting  solution 
analyzed;  but  it  was  found  that  even  a i per  cent,  solution  of 
KCN  will  attack  bornite  to  a degree  which  is  sufficient  to  render 
this  method  of  analysis  useless;  thus  preventing  the  use  of  an 
extremely  helpful  analytical  method.72  It  was  also  found  diffi- 
cult to  obtain  a sufficient  amount  of  pure  bornite  to  carry  out  our 
work.  The  material  studied  was  the  purest  obtainable.  The 
analyses  of  the  various  samples  of  bornite  used  in  our  experi- 
ments are  tabulated  below  :73 

TABLE  XXIX. 

Analyses  of  Bornite. 


Locality. 

Cu. 

Fe. 

•s. 

Pb. 

A g. 

Superior,  Ariz 

Costa  Rica 

62.99 

62.99 

II.23 

11.20 

25.58 

25-54 

O.IO 

Unknown,  (Z) 

Calculated  for  (Cu5FeS4)74 

63.19 

63.33 

II.31 

II. 12 

25-44 

25-55 

.02 

A.  Bornite  and  Sulphuric  Acid. 

While  trying  to  purify  some  bornite  we  found  that  this  sul- 
phide is  rather  sensitive  to  sulphuric  acid,  especially  so  at  elevated 
temperatures.  It  was  therefore  advisable  to  obtain  an  idea  as  to 
the  products  formed.  The  initial  conditions  and  duration  of  these 
experiments  are  tabulated  below : 

72  The  correction  for  solubility  of  the  bornite  is  large  and  at  present  can 
not  be  applied  with  certainty. 

73  E.  T.  Allen,  Am.  I.  Sci.,  XLI.,  409-413  (1916). 

74  Idem,  p.  410. 


476 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


TABLE  XXX. 

Initial  Conditions  and  Duration  of  Experiments. 


Exp. 

Weight, 

g- 

Temp. 

Solution. 

Duration. 

52 

Bornite  from  Superior 

Ariz.,  small  lump  and 

100  mesh  and  finer 

1.0 

200° 

12.5  C.C.  2\%  H2SO4.  . 

2 days 

53 

Bornite  from  Costa  Rica, 

lumps  and  finer 

200° 

12.5  C.C.  2%  H2SO4.  . . 

8 days 

54 

Bornite  from  Costa  Rica, 

125—200  mesh76 

I. OOO 

100° 

50  C.C.  2%  H2SO4 

2 days 

55 

Bornite  from  Superior 

Ariz.,  125-200  mesh76  . . 

4.000 

O 

0 

400  C.C.  I % H2SO4 

2 months 

Products  Formed  at  200° . — The  residue  in  52  presented  quite 
a different  appearance  from  the  original  bornite  and  on  being 
examined  microscopically  was  shown  to  be  made  up  as  follows : 
Twinned  cubes  of  cuprous  sulphide  were  found  growing  on  the 
lump,  the  fine  powder  had  a grayish  blue  appearance  suggesting 
the  presence  of  cuprous  together  with  cupric  sulphide ; the  lump 
was  broken  in  two,  polished,  and  examined  microscopically.  This 
examination  revealed  the  presence  of  chalcopyrite  in  thin  lines 
seemingly  following  fracture  and  cleavage  directions.  This  bor- 
nite had  been  examined  in  a similar  manner  before  treatment  with 
acid  and  no  chalcopyrite  found.  In  addition  to  the  copper  sul- 
phides and  chalcopyrite,  the  presence  of  very  fine  crystals  of  one 
or  both  disulphides  of  iron  was  noted.  The  solution  contained 
ferrous  sulphate  and  hydrogen  sulphide.  We  have  here  a very 
complicated  reaction,  the  detailed  study  of  which  would  lead  us 
beyond  the  scope  of  this  paper.  The  formation  of  the  chalco- 
pyrite within  the  lump  of  bornite  was  studied  in  a qualitative 
manner  and  will  be  dealt  with  in  a subsequent  paper  on  which  we 
hope  to  report  shortly.  The  evidence  brought  out  by  this  study 
led  us  to  the  following  explanation  for  these  interesting  facts : 
The  acid  first  attacked  the  surface  of  the  bornite  altering  it  to 
cupric  and  cuprous  sulphides ; ferrous  sulphate  and  hydrogen  sul- 
phide being  formed  at  the  same  time.  The  interior  thus  became 
protected  from  the  direct  action  of  the  relatively  strong  acid  and 
75  See  page  410. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4 77 


the  small  amount  of  acid  which  did  penetrate  was  greatly  weak- 
ened by  reacting  with  the  bornite.  Under  these  conditions  we 
have  found  that  ferrous  sulphate,  cupric  or  cuprous  sulphide,  and 
hydrogen  sulphide  will  react  and  form  chalcopyrite.  The  most 
likely  place  for  the  chalcopyrite  to  develop  will  be  of  course  along 
the  lines  of  fracture  and  cleavage  since  they  afford  to  the  acid  the 
readiest  means  of  ingress. 

The  appearance  of  the  residue  in  Experiment  53  was  much  the 
same  as  that  observed  in  52  and  the  same  formation  of  chalco- 
pyrite within  the  lump  was  noted. 

In  experiment  54,  carried  out  at  ioo°,  the  appearance  of  the 
residue  was  again  markedly  different  from  that  of  the  original 
bornite.  The  surface  of  the  grains  was  coated  over  with  what 
the  microscope  proved  to  be  covellite  and  chalcocite,  giving  the 
residue  a bluish  appearance.  Ferrous  sulphate  and  hydrogen 
sulphide  were  found  in  solution  in  approximately  the  following 
amounts:  Fe  = 0.039 g.  and  H2S  = 0.023  g.  The  exact  values 
are  of  course  not  significant,  but  the  figures  show  that  bornite  is 
appreciably  attacked  by  sulphuric  acid  at  temperatures  where 
pyrite  and  chalcopyrite,  similarly  sized,  show  no  action.76  In 
order  to  compare  the  action  of  acid  on  the  sulphides,  it  is  obvious 
that  the  sulphides  should  be  carefully  sized  so  as  to  present  com- 
parable surfaces.77 

At  40°,  bornite  is  also  markedly  attacked  by  sulphuric  acid; 
not  as  easily,  to  be  sure,  as  at  ioo°,  yet  at  the  end  of  2 months, 
during  which  the  bornite,  sized  as  indicated,  had  been  exposed  to 
the  action  of  a 1 per  cent,  solution  of  sulphuric  acid,  0.0236  g.  of 
iron  in  the  ferrous  condition  was  found  in  solution,  a quantity 
which  must  have  been  derived  from  the  alteration  of  0.2120  g.  of 
bornite.  Hydrogen  sulphide  was  very  easily  recognized  by  its 

76  See  page  465. 

77  For  work  along  similar  lines,,  see  J.  D.  Clark,  Bull.  Univ.  of  Mex.,  75, 
11 6;  also  referred  to  in  Tolman  and  Clark,  Econ.  Geol.,  IX.,  570.  No  close 
sizing  was  attempted  in  the  experiments  carried  out  by  Clark,  hence  it  is 
somewhat  difficult  to  understand  how  the  results  obtained  in  his  experiments 
are  comparable  with  one  another.  Thus  the  200-mesh  pyrite  may  have  con- 
tained more  very  fine  flour  than  the  200-mesh  bornite. 


478 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


odor.78  The  residue  at  40°  had  the  same  bluish  appearance 
noted  in  the  case  of  bornite  treated  in  the  same  manner  at  ioo°. 

B.  Bornite  and  Cupric  Sulphate  at  200°  C. 

(a)  The  Alteration  of  Bornite  to  the  Sulphides  of  Copper. 

In  the  preliminary  experiments  carried  out  in  studying  this 
reaction,  it  was  found  that  so  little  acid  is  formed  when  one  gram 
of  bornite  is  treated  with  25  c.c.  of  a 5 per  cent,  cupric  sulphate 
solution  that  the  hematite  resulting  from  the  reaction  between  fer- 
rous sulphate  and  cupric  sulphate  persists  for  a long  time — as  a 
matter  of  fact,  in  experiment  57,  tabulated  below,  which  was  car- 
ried out  with  the  idea  of  converting  the  bornite  completely  into 
chalcocite,  some  hematite  persisted  even  at  the  end  of  one  month’s 
time.  In  experiment  55,  the  surface  of  the  bornite  was  increased 
by  using  a larger  amount  of  bornite  ground  to  pass  through  a 1 oo- 
mesh  bolting  cloth.  When  this  large  surface  was  made  as  avail- 
able as  possible  by  mixing  the  bornite  with  three  times  its  volume 

TABLE  XXXI. 

Bornite  and  Cupric  Sulphate. 


Temperature,  200° . 


Initial  Conditions. 

Analyses  of  Solutions. 

Exp. 

Duration. 

Weight, 

g- 

Copper,  g 

Acid, 

Total  Iron, 
g. 

Material. 

Solution. 

Initial. 

Final. 

Depos- 

ited. 

H2SO4, 
S ■ 

56 

i day 

5.000 

Bornite, 
Superior, 
Ariz.,  100 
mesh  and 
finer. 

50  c.c.  5% 

CuSC>4.- 

5H2O 

O.6363 

0.0040 

O.6323 

O.0963 

O.501579 

57 

i month 

I. OOO 

Bornite, 
Superior, 
Ariz.,  100 
mesh  and 
finer. 

50  c.c. 
abt.  2% 
CuS04.- 
5H2O 

0.2657 

0.0135 

0.2522 

O.3040 

O.IO4480 

1 

78  Of  course,  in  the  presence  of  cupric  sulphate  together  with  the  acid,  no 
odor  of  H2S  would  be  noticed. 

79  All  ferrous. 

80  0.0122  as  hematite. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  479 


of  pure  quartz,  no  hematite  was  formed,  and  the  copper  in  solu- 
tion was  used  up  very  quickly. 

In  experiment  57  hematite  persisted  even  after  the  lapse  of  one 
month.  Its  presence  indicates  the  hydrolysis  of  some  ferric  salt 
the  formation  of  which  has  been  several  times  pointed  out  in 
similar  cases.  The  molecular  ratios  obtained  in  experiment  57 
are  affected,  though  only  to  a slight  degree,  by  this  secondary 
action. 


Molecular  Ratios  Based  on  Analyses  of  the  Solutions. 


Exp. 

Cu 

H2SO4 

Cu 

Fe‘ 

Fe  ‘ 

h2so4‘ 

56 

I. II 

O.II 

10. 1 

57 

2.12 

HI 

1.3  

These  ratios  bring  out  some  rather  striking  differences  in  the  two 
experiments  and  will  be  discussed  together  with  the  microscopic 
evidence  derived  from  the  examination  of  the  residues. 

Microscopic  Examination. — These  residues  were  compressed 
as  usual,  polished,  and  examined.  The  color  of  each  was  com- 
pared with  that  of  a pure  specimen  of  chalcocite.  In  experiment 
56,  both  cupric  and  cuprous  sulphide  were  observed  as  distinct 
constituents.  In  addition,  the  color  of  the  cuprous  sulphide  was 
darker  than  that  of  the  pure  mineral  indicating  the  presence  of 
cupric  sulphide  in  solid  solution.81  This  residue  was  also  ex- 
amined after  imbedding  a few  of  the  grains  in  sealing  wax,  and 
polishing  deeply  into  the  grains.82  In  addition  to  the  sulphides 
of  copper,  small  blades  of  chalcopyrite  were  found,  seemingly 
following  the  cleavage  cracks  in  the  original  bornite.  This  bor- 
nite  had  been  examined  before  the  experiment,  and  no  chalco- 
pyrite found.  It  will  be  remembered  that  sulphuric  acid,  also, 
caused  an  indirect  formation  of  chalcopyrite  within  the  bornite,83 
and  since  we  know  that  acid  was  present  at  the  end  of  the  experi- 
ment, the  presence  of  this  chalcopyrite  is  accounted  for.  The 

81  Posnjak,  Allen  and  Merwin,  Econ.  Geol.,  X.,  506  (1915). 

82  This  examination  was  made  by  Professor  Graton. 

83  See  page  477. 


480 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


amount  of  the  chalcopyrite,  however,  could  not  have  been  more 
than  a fraction  of  a per  cent.  In  experiment  5 7,  the  color  of  the 
residue  matched  that  of  the  standard  chalcocite  very  closely,  and 
it  was  therefore  concluded  that  the  bornite  in  this  case  had  been 
altered  to  chalcocite. 

( b ) Discussion  of  Results. 

The  results  of  the  microscopic  examination  obtained  in  experi- 
ment 57,  taken  together  with  the  molecular  ratios,  indicate  that 
the  reaction  progresses  according  to  the  following  equation : 

5Cu5FeS4  -f- 1 iCuS04  + 8H20  = i8Cu2S 

+ 5FeS04  + 8H2S04.  (9) 


The  molecular  ratios  demanded  by  this  equation  and  those  ob- 
tained by  experiment  are  tabulated  below : 


Cu  Deposited 

h2so4 

Cu  Deposited 

' Fe  ' 

Fe 

H2S04 

By  equation  (9) 

2.2 

I.60 

1-375 

By  experiment  57 

2.12 

1-7 

1-3 

The  agreement  is  as  close  as  can  be  expected,  considering  the 
experimental  difficulties  involved. 

In  experiment  56,  both  cupric  and  cuprous  sulphide  were  found 
in  rather  large  amounts  together  with  a little  chalcopyrite.  As 
well  as  could  be  determined  all  of  the  bornite  had  altered  to  these 
substances.  No  hematite  was  observed,  and  the  amounts  of 
copper,  iron,  and  acid  were  such  that  they  could  be  determined 
with  great  accuracy.  There  is  an  error  introduced  by  the  forma- 
tion of  a little  chalcopyrite,  but  its  magnitude  must  be  very  small 
since  the  amount  of  this  substance  was  very  slight.  Even  though 
the  acid  does  attack  the  bornite,  forming,  as  we  have  seen,  ferrous 
sulphate  and  hydrogen  sulphide,  yet  in  the  presence  of  cupric  sul- 
phate all  of  the  acid  used  up  in  this  manner  is  regenerated  owing 
to  the  action  of  the  hydrogen  sulphide  on  the  cupric  sulphate. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  481 


We  infer,  therefore,  that  the  ratios  calculated  on  the  basis  of  the 
analysis  of  the  solution  are  accurate. 

The  molecular  ratios  obtained  in  these  experiments  show  that 
for  a given  amount  of  copper  deposited,  less  acid  was  formed  in 
experiment  56  than  in  57,;  the  microscopic  examination  showed 
in  57  that  the  bomite  had  altered  into  chalcocite,  but  that  in  56 
both  cupric  and  cuprous  sulphide  were  present.  It  is  evident  then 
that  when  cupric  sulphide  is  present  less  acid  is  formed.  The 
molecular  ratios  differ  so  greatly  in  the  two  experiments  that  we 
are  led  to  the  conclusion  that  if  the  surface  of  the  bornite  were 
large  enough  to  react  at  once  with  all  of  the  copper  present  in 
solution,  no  acid  whatsoever  would  have  been  formed  and  that 
the  acid  which  was  formed  was  derived  from  the  reaction  be- 
tween cupric  sulphide  and  cupric  sulphate.  If  we  assume  that 
the  acid  was  derived  as  indicated,  then,  knowing  the  amount  of 
acid  formed,  we  should  be  able  to  calculate  on  the  basis  of  equa- 
tion ( 1 ) the  amount  of  copper  which  reacted  with  cupric  sulphide 
to  form  this  acid.  Acid  to  the  amount  of  0.0963  g.  was  found  in 
experiment  56.  The  formation  of  this  amount  must  have  been 
accompanied  by  the  deposition  of  0.0468  g.  of  copper  which  re- 
acted with  the  cupric  sulphide  to  form  cuprous  sulphide.  De- 
ducting this  from  the  total  copper  deposited,  we  obtain  the  amount 
of  copper  which  reacted  directly  with  the  bornite,  namely, 
0.5855  g.  The  0.5015  g.  of  iron  found  in  solution  was  formed 
during  this  reaction.  The  molecular  ratio  Cu/Fe  calculated 
from  these  data  is  1.03  g.  This  ratio  agrees  well  with  that  de- 
manded by  equation  (10),  namely,  unity. 

Cu5FeS4  + CuS04  = 2Cu2S  + 2CuS  + FeS04.  ( 10) 

Furthermore,  1 g.  of  the  residue  obtained  in  experiment  56  was 
analyzed  and  yielded'  the  following  results,  corrected  for  the 
silica  present  in  the  residue : 

Cu  — 0.7414,  Fe=:o.oioi,  S = undetermined. 


482 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


The  iron  found  in  the  residue  is  attributed  in  part  to  the  chalco- 
pyrite,  whose  presence  in  the  residue  was  revealed  by  the  micro- 
scopic examination,  and  in  part  to  unattacked  bornite.  Assum- 
ing that  all  the  iron  was  derived  from  chalcopyrite  (CuFeS2), 
we  find  by  calculation  that  this  amount  of  iron  is  equivalent  to 
0.0332  g.  CuFeS2.  This  amount  of  CuFeS2  contains  0.0115  g. 
Cu.  Deducting  the  weight  of  the  CuFeS2  from  1.0000  g.,  the 
total  weight  of  the  residue,  and  deducting  the  amount  of  Cu  in 
the  CuFeS2  from  the  total  Cu  found  by  analysis,  we  obtain  by 
calculation  75.5  per  cent.  Cu.  This  is  the  percentage  of  Cu  pres- 
ent in  the  residue  as  sulphides  of  copper.  We  find,  by  using  this 
percentage  as  the  basis  of  our  calculation,  that  the  residue  con- 
tained 60  per  cent,  of  Cu2S  and  40  per  cent,  of  CuS.  Knowing 
the  amount  of  acid  formed  in  experiment  56  and  the  total  amount 
of  copper  which  reacted,  we  can  calculate  on  the  basis  of  equa- 
tions (10)  and  (1),  assuming  for  the  present  the  correctness  of 
(10),  the  amount  of  Cu2S  and  CuS  which  should  be  present  in 
the  residue.  Thus  we  obtain  65  per  cent.  Cu2S  and  35  per  cent. 
CuS.  In  view  of  the  uncertainty  involved  in  making  the  proper 
correction  for  the  iron  found  by  actual  analysis  of  the  residue, 
and  in  view  of  the  experimental  errors  involved,  the  agreement 
between  the  amounts  of  Cu2S  and  CuS  just  calculated  with  those 
calculated  from  the  analysis  is  as  good  as  can  be  expected. 

Thus  the  analysis  of  the  residue  furnishes  additional  evidence 
in  favor  of  equation  (10)  as  representing  one  of  the  stages  of  the 
reaction  when  bornite  reacts  with  cupric  sulphate. 

It  would  have  been  desirable  to  carry  out  other  similar  experi- 
ments with  still  larger  quantities  of  finely  ground  bornite,  but 
we  were  prevented  from  doing  so  by  the  scarcity  of  the  pure 
mineral. 

C.  Bornite  and  Cupric  Sulphate  at  ioo°. 

The  one  experiment  carried  out  at  this  temperature  is  tabulated 
below ; the  bornite  was  mixed,  as  usual,  with  three  times  its  vol- 
ume of  pure  quartz. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4^3 

TABLE  XXXII. 

Bornite  and  Cupric  Sulphate. 

Initial  Conditions:  1.000  g.  bornite  from  Superior,  Ariz.  (ioo  mesh  and  finer)  ; 
solution,  50  c.c.  1 per  cent.  CuS04-5H20;  temperature,  ioo0. 


Analysis  of  Solution. 

Exp. 

Duration, 

Months. 

Copper,  g. 

Total  Iron  ; 

Acid, 

Initial. 

Final.  Deposited. 

( Ferrous),  g. 

H2SO4,  g. 

58 

I 

O.I265 

None  0.1265 

O.O97484 

O.0357 

Molecular  ratios  based  on  this  analysis : 

Cu  H2S04  Cu 

Fe  - I-I4:  Fe  ' ~ 0-21  ’ H2S04  “ 5'47' 

The  residue  obtained  in  this  experiment  was  compressed  and  ex- 
amined microscopically.  The  presence  of  both  cupric  and  cu- 
prous sulphides  was  noted.  When  we  compare  the  ratios  obtained 
in  this  experiment  with  those  obtained  when  bornite  alters  to 
cuprous  sulphide  only,  we  see  that  here  again  for  a given  amount 
of  copper  deposited  as  sulphide  on  the  bornite  less  acid  is  formed 
when  both  cupric  and  cuprous  sulphides  are  present  than  when 
cuprous  sulphide  alone  is  present : 


J 

Cu  Deposited 

H2SO4 

Cu  Deposited 

Fe 

Fe 

H2SO4 

Exp.  57  at  200  (all  chalcocite) 

2.12 

1-7 

1-3 

Exp.  58  at  100 

1. 14 

0.21 

5-47 

The  ratios  obtained  in  the  experiment  at  ioo°  also  approach 
the  values  demanded  by  equation  (xo),  but  not  quite  so  closely 
as  in  the  case  of  56  at  200°.  If  we  deduct  the  small  amount  of 
copper  involved  in  the  formation  of  the  acid,  namely,  through 
equation  (1),  and  then  calculate  the  molecular  ratio  of  the  re- 
maining copper  to  iron  in  the  same  way  as  before,  the  ratio  be- 
comes 1 : 1 as  demanded  by  equation  ( 10). 

84  A very  little  hematite  remained  attached  to  the  wall  of  the  tube  and 
amounted  to  about  1 milligram. 


484 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


D.  Bornite  and  Cupric  Sulphate  at  Ordinary  Temperatures. 

These  experiments  were  carried  out  in  the  manner  previously 
described.85  Inasmuch  as  the  bottles  were  shaken  throughout 
the  experiment,  no  quartz  was  required  to  facilitate  the  circulation 
of  liquid  through  the  mass  of  the  mineral — an  absolute  necessity 
when  the  experiments  were  done  in  sealed  tubes. 


TABLE  XXXIII. 
Bornite  and  Cupric  Sulphate. 
Initial  Conditions. 


Exp. 

Material. 

Weight, 

g- 

Temp. 

Solution. 

59 

Bornite  from  Superior,  Ariz., 
100  mesh  and  finer 

3.000 

30  ±5 

200  C.C.  l\%  CUSO4.5H2O 

60 

Bornite  from  Superior,  Ariz., 
125-200  mesh 

6.000 

30  ± 5 

400  C.C.  Ij%  CUSO4.5H2O 

TABLE  XXXIV. 
Analyses  of  the  Solutions. 


Exp. 

Copper,  G. 

Total  Iron. 

Acid. 

Duration. 

Initial. 

Final. 

Deposited. 

59 

60 

0.6407 

I.2718 

0.5405 

I.2495 

0.1002 

0.0223 

0.0875 

O.OI97 

Trace86 
“ 88 

1 month 

2 months 

The  copper  and  the  iron  were  determined  in  the  total  filtrate 
from  the  bornite  in  the  same  manner  as  indicated  in  the  experi- 
ments with  chalcopyrite.  It  will  be  noted  that  more  copper  was 
deposited  in  59  than  in  60  even  though  the  weight  of  bornite 
was  greater  in  60  than  59.  Thus  in  59,  3 grams  of  bornite  were 
ground  to  pass  through  a 100-mesh  bolting  cloth;  of  this  amount, 
48  per  cent,  passed  through  a 200-mesh  sieve.  Thus  all  of  the 
fine  flour  was  included.  In  60,  the  6 grams  of  bornite  were  care- 
fully sized  between  125  and  200  mesh  and  the  adhering  fine  flour 
removed  by  elutriation.87  It  is  therefore  believed  that  the  dif- 

85  See  page  415. 

86  Very  faintly  acid  to  methyl  orange.  The  color  of  the  solution  makes  it 
impossible  to  determine  very  small  amounts  of  acid. 

87  See  page  410. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT . 4^5 

ference  in  the  amounts  of  copper  deposited  in  the  two  experi- 
ments is  due  in  part  at  least  to  the  difference  in  the  amount  of 
surface  of  bornite  exposed  to  the  action  of  cupric  sulphate. 

The  molecular  ratio  Cu/Fe  was  determined  for  each  experi- 
ment: Cu/Fe  in  experiment  59=1.00  and  in  60=1.0.  The 
residue  in  59  was  examined  microscopically  and  the  presence  of 
cupric  and  cuprous  sulphide  determined.88’ 89  This  evidence 
taken  together  with  the  evidence  of  the  ratios  at  temperatures 
ranging  from  30°  to  200°,  but  especially  at  30°  again  point  to 
the  probability  of  the  reaction  represented  by  fhe  following 
equation : 

Cu5FeS4  -f  CuS04  = 2Cu2S  + 2CuS  -f-  FeS04  (10) 

, . , , . Cu  deposited 

in  which  the  ratio  _ = 1. 

Fe 

(a)  The  Influence  of  Sulphuric  Acid. 

The  following  experiments  were  carried  out  at  the  same  time 
and  under  the  same  conditions  in  order  to  determine  the  influence 
of  sulphuric  acid  on  the  reaction  between  bornite  and  sulphuric 
acid. 

TABLE  XXXV. 

Bornite  and  Cupric  Sulphate.  Influence  of  Sulphuric  Acid. 


Initial  Conditions:  4.000  g.  bornite  No.  2 from  Superior,  Ariz.90  (125-200 
mesh)  ; temperature,  30  + 50. 


Exp. 

Dura- 

tion, 

Months. 

Initial  Concentration 
of  So  ution. 

Analyses  of  Sol 

Copper,  g. 

utions. 

Acid  , 
Initial. 

Total 
Iron, 
g ■ 

Initial. 

Final. 

Depos- 

ited. 

6l 

2 

400  C.C.  1%  CUSO4.5H2O 

1.0227 

1.0182 

O.OO45 

None 

O.OO43 

62 

2 

400  c.c.  1%  CUSO4.5H2O  and 

1%  H2SO4 

1.0227 

1.0078 

O.OI49 

1% 

O.OI7I 

63 

2 

400  C.C.  I % H2SO4 

None 

None 

None 

1% 

O.O236 

and 

h2s 

88  Examined  by  Professor  Graton. 

89  This  was  not  done  in  the  case  of  the  second  experiment  as  not  enough 
bornite  had  altered  to  make  this  examination  worth  while. 

90  This  bornite  contained  about  8 per  cent.  CtuS  and  was  the  purest  available 
when  these  experiments  were  carried  out. 


486 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


We  do  not  wish  to  lay  any  stress  on  the  absolute  values  shown 
above,  but  rather  on  the  relative  values;  they  vary  sufficiently  to 
make  a comparison  worth  while.  It  is  evident  from  the  amount 
of  copper  deposited  that  the  acid  exerted  an  accelerating  influ- 
ence. We  see  from  the  amount  of  iron  in  solution  that  more 
bornite  was  altered  when  I per  cent,  acid  alone  was  used  than 
when  i per  cent,  acid  together  with  cupric  sulphate  was  used. 
This  is  presumably  due  to  the  protective  action  exerted  by  the 
firmly  adhering  alteration  products  formed  on  the  bornite. 

,IO.  THE  REACTION  BETWEEN  SPHALERITE  (ZnS)  AND  CUPRIC 

SULPHATE. 

A.  The  Reaction  at  200° . 

The  sphalerite  used  in  our  experimental  work  was  carefully 
selected  from  material  obtained  from  two  sources,  namely,  from 
Sonora,  Mexico,  and  from  Joplin,  Missouri.  These  samples  of 
sphalerite  were  analyzed  for  impurities. 


Zn. 

Fe. 

S. 

Summation. 

Sonora,  Mexico 

66.98 

O.I5 

•23 

32.78 

99.91 

Joplin,  Missouri 

Both  the  microscopic  examination  and  the  chemical  analysis 
showed  that  both  samples  of  sphalerite  were  very  pure.  Experi- 
ments were  carried  out  at  200°  and  at  ordinary  temperatures. 


TABLE  XXXVI. 

Sphalerite  and  Cupric  Sulphate. 

Initial  Conditions:  Material,  sphalerite  from  Sonora,  Mexico  (100  mesh  and 
finer)  ; solution,  20  c.c.  2 per  cent.  CuS04-5H20;  temperature,  200°. 


Exp. 

Duration, 

Days. 

Weight, 
g ■ 

Analyses  of  Solutions. 

Copper,  g. 

Zinc  in 
Solution, 
g • 

Acid, 

H2SO4, 

g. 

Initial. 

Final. 

Deposited. 

64 

65 

2 

2 

2.000 

1.300 

0.1030 

0.1032 

None 

None 

0.1030 

O.IO32 

O.089O 

O.0858 

0.0361 
Not  det. 

SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4%  7 


It  is  evident  from  these  analyses  that  copper  sulphate  and 
sphalerite  react,  copper  being  deposited  and  zinc  dissolved,  while 
sulphuric  acid  is  also  formed. 

Microscopic  Examination. — On  examining  the  solid  products 
microscopically,  it  was  found  that  copper  had  been  deposited  as 
sulphides  partly  replacing  the  sphalerite.  In  all  probability  both 
cupric  and  cuprous  sulphides  were  present,  as  indicated  by  the 
color  of  the  grains  of  sphalerite,  which  were  coated  with  firmly 
adhering  blue  and  bluish  gray  films.  Furthermore,  it  was  found 
that  some  of  the  grains  were  covered  with  films  so  thin  as  to 
transmit  the  green  light  characteristic  of  covellite. 

The  precipitated  copper  sulphides  were  then  analyzed  chem- 
ically in  the  usual  manner.  If  a i per  cent,  solution  of  potas- 
sium cyanide  is  employed  to  dissolve  the  copper  sulphides,  and 
the  solution  and  washing  of  the  residues  are  rapidly  done,  the 
solvent  action  of  the  reagent  on  the  sphalerite  is  negligible. 
Sphalerite,  sized  between  125  and  200  mesh,  thus  behaves  quite 
differently  from  precipitated  zinc  sulphide,  for  the  latter  is 
readily  attacked  by  even  a 1 per  cent,  solution  of  potassium  cya- 
nide. The  following  analyses  of  the  enrichment  products  replac- 
ing the  sphalerite  were  carried  out  in  the  manner  just  indicated: 


TABLE  XXXVII. 
Sphalerite  and  Cupric  Sulphate. 
Analyses  of  the  Alteration  Products. 


Exp. 

Cu  in  KCN,  g. 

S in  KCN,  g. 

Total,  g. 

Per  Cent, 
of  Cu. 

Per  Cent, 
of  CU2S. 

Per  Cent, 
of  CuS. 

64 

0.0663 

0.0253 

0.0916 

73.38 

44  ± 3 

56 

65 

0.0675 

0.0246 

O.0931 

73-29 

51  ± 3 

49 

These  analyses  confirm  the  observation  made  with  the  micro- 
scope, namely,  that  cupric  and  cuprous  sulphide  are  present  in 
the  enrichment  products.  Furthermore,  the  analysis  of  the  solu- 
tion in  experiment  64  showed  the  presence  of  sulphuric  acid, 
which  was  no  doubt  derived  from  the  conversion  of  some  of  the 
cupric  sulphide  into  cuprous  sulphide,  a reaction  which  is  com- 
paratively rapid  at  200°.  On  this  assumption,  we  may,  as  usual, 


488 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


calculate  the  amount  of  cupric  and  cuprous  sulphide  which  should 
be  present  from  the  amount  of  copper  deposited  and  the  amount 
of  acid  formed.  In  experiment  64,  0.0361  gr.  of  acid  was 
found.  This,  on  the  basis  of  equation  (1),  is  equivalent  to 
0.0586  grams  of  Cu2S,  which  in  turn  contains  0.0468  grams  of 
copper.  This  amount  of  copper,  when  deducted  from  the  total 
copper  deposited,  namely,  0.1030  grams,  gives  the  amount  of 
copper  present  as  cupric  sulphide,  or  0.0562.  This  amount  of 
copper  is  contained  in  0.0846  grams  of  CuS.  Such  a mixture 
contains  41  per  cent.  Cu2S  and  59  per  cent.  CuS,  while  we  found 
by  analysis  44  per  cent,  of  Cu2S  and  56  per  cent,  of  CuS. 

On  the  basis  of  equation  ( 1 ),  we  can  also  determine  the  amount 
of  copper  which  reacted  with  the  cupric  sulphide  to  form  cuprous 
sulphide,  knowing  the  amount  of  sulphuric  acid  formed.  Mak- 
ing this  calculation  for  experiment  64,  we  find  that  0.0176  grams 
of  copper  reacted  in  this  manner.  This  amount  deducted  from 
the  total  copper  deposited  gives  the  amount  of  copper  which 
reacted  with  the  sphalerite  to  form  cupric  sulphide.  We  find  this 
amount  to  be  0.0854  grams.  When  we  calculate  the  molecular 
ratio  of  this  copper  to  that  of  the  zinc  found  in  solution,  we 
obtain  Cu/Zn  = o.99.  This  value  points  to  the  following  equa- 
tion as  representing  the  first  reaction  which  taken  place  between 
sphalerite  and  cupric  sulphate: 

ZnS  + CuS04  = CuS  + ZnS04.  (11) 

The  question  will  no  doubt  be  asked  how  can  the  acid  formed  in 
the  reaction  persist  in  the  presence  of  sphalerite.  The  solution  in 
experiment  64  was  carefully  examined  for  hydrogen  sulphide, 
according  to  the  method  previously  described,91  but  none  was 
found.  So  long  as  copper  is  present  in  solution,  any  hydrogen 
sulphide  that  may  be  formed  by  the  action  of  the  acid  on  the 
sphalerite  is  used  up  in  precipitating  sulphide  of  copper  and  at 
the  same  time  setting  free  the  same  amount  of  acid  as  was  used 
up  in  attacking  the  sphalerite.  Therefore,  so  long  as  copper  is 
present  in  solution,  so  long  will  the  amount  of  acid  formed  accord- 
ing to  experiment  64  persist.  Later  on  when  all  of  the  copper 

91  See  page  445. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  4% 9 


has  been  deposited,  as  was  the  case  in  64,  the  coating  of  sulphides 
of  copper  formed  on  the  sphalerite  protects  the  latter,  and  if  the 
tube  is  removed  from  the  furnace  as  soon  as  all  of  the  copper  is 
deposited,  little  if  any  of  the  acid  is  used  up. 

In  the  second  experiment  on  cupric  sulphate  and  sphalerite, 
acid  was  likewise  formed,  though  it  was  not  determined.  Never- 
theless, here  also  we  may  calculate  the  amounts  of  cuprous  and 
cupric  sulphides  formed,  using  as  data  the  quantity  of  zinc  dis- 
solved and  the  quantity  of  copper  precipitated,  if  we  assume  equa- 
tions (1)  and  (11),  which  we  are  now  perfectly  justified  in 
doing.  In  this  way,  we  obtain  46  per  cent,  of  Cu2S  and  54  per 
cent  of  CuS ; the  agreement  with  the  amounts  found  by  analysis 
of  the  coating  on  the  sphalerite  is  again  as  close  as  the  experi- 
mental error  involved  will  permit,  especially  in  view  of  the  fact 
that  a very  faint  test  for  hydrogen  sulphide  was  obtained  in  this 
experiment,  showing  that  the  acid  which  formed  during  the  ex- 
periment attacked  the  sphalerite  after  all  copper  had  been  de- 
posited. This  will  make  the  amount  of  cupric  sulphide  calculated 
on  the  basis  of  the  zinc  in  solution  higher  than  when  no  hydrogen 
sulphide  is  present  finally.  Comparing  the  calculated  amount  of 
cupric  sulphide  with  that  found  by  analysis,  we  note  that  such  is 
the  case. 

B.  Sphalerite  and  Cupric  Sulphate  at  Ordinary  Temperatures. 

The  experiments  at  ordinary  temperatures  were  carried  out 
under  the  conditions  previously  described.92 


TABLE  XXXVIII. 
Sphalerite  and  Cupric  Sulphate. 


V 

Initial  Conditions. 

A 

nalyses  of  Solutions. 

d 

J-S 

« a 

g 

<L> 

Weight, 
S ■ 

Copper,  g. 

Zinc, 

g- 

W 

- 0 

a 

a 

V 

H 

Material. 

Solution. 

Initial. 

Final. 

Depos- 

ited. 

66 

2 

30  ± 5 

4.881 

Sonora, 
Mexico, 
125-200  mesh 

400  C.C.  l\% 

C11SO4.- 

5H2O 

I.2718 

1. 2112 

O.0618 

0.0515 

67 

2 

40  ± 1 

12.000 

Joplin,  Mo., 
125-200  mesh 

200  C.C.  l\% 

C11SO4  .- 
5H2O 

0.6359 

O.54OI 

0.0958 

O.0855 

92  See  page  415. 


490 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


Sulphuric  acid  was  present  in  both  solutions,  but  the  amount 
was  too  small  to  determine  with  any  accuracy.93  The  residues 
were  examined  microscopically  but  the  films  of  the  enrichment 
products  which  partly  replaced  the  sphalerite  were  too  thin  for 
identification.  These  films  were  removed  with  a I per  cent,  solu- 
tion of  potassium  cyanide  and  analyzed  in  the  usual  manner. 


TABLE  XXXIX. 
Sphalerite  and  Cupric  Sulphate. 
Analyses  of  the  Enrichment  Products. 


Exp. 

Copper  in 
KCN,  g. 

S in  KCN.  g. 

Total,  g. 

Per  Cent, 
of  Cu. 

Per  Cent, 
of  Cu2S. 

Per  Cent, 
of  CuS. 

66 

O.0616 

0.0228 

0.0844 

73-0 

49  ± 5 

51 

67 

0.0572 

0.0234 

0.0806 

71.0 

34  ± 5 

66 

The  experimental  evidence  at  200°  showed  that  sphalerite  and 
cupric  sulphate  reacted  to  form  first  cupric  sulphide,  and  that 
this  sulphide  in  turn  reacted  with  the  cupric  sulphate  according 
to  equation  (i)  to  form  cuprous  sulphide.  The  presence  of  sul- 
phuric acid  in  the  solution  and  the  presence  of  cuprous  sulphide 
in  the  enrichment  product  on  the  sphalerite  indicate  that  the  same 
reactions  take  place  at  the  lower  temperatures.  If  this  is  the  case, 
we  should  be  able  to  calculate,  on  the  basis  of  equations  (n) 
and  (i),  the  amount  of  cuprous  and  cupric  sulphides  present  at 
the  end  of  the  experiment,  if  we  know  the  amount  of  copper  lost 
by  the  solution  during  the  experiment  and  the  amount  of  zinc 
found  in  solution.  The  calculation  was  made,  and  the  results 
are  tabulated  below: 


Exp. 

Calculated. 

Analyzed. 

Per  Cent,  of  Cu2S. 

Per  Cent,  of  CuS. 

Per*  Cent,  of  Cu2S. 

Per  Cent.1  of  CuS. 

66 

46 

54 

49  ± 5 

51 

67 

31 

69 

34  ± 5 

66 

The  agreements  are  within  the  limits  of  experimental  error. 


(a)  The  Influence  of  Sulphuric  Acid. 

The  sphalerite  used  in  these  comparison  experiments  was 
carefully  sized. 

93  See  page  437. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  49 1 


TABLE  XL. 

Initial  Conditions:  12.000  g.  sphalerite  from  Joplin,  Mo.  (125-200  mesh); 
temperature,  40° ; duration  of  experiments,  two  months. 


Analyses  of  the  Solutions. 


Exp. 

Solution. 

Copper,  g. 

Zinc  in 
Solu- 
tion. 

Acid,  g. 
Initial. 

Initial. 

Final. 

Depos- 

ited. 

67 

200  C.C.  l\%  CUSO4.5H2O 

O.6359 

O.5401 

0.0958 

O.0855 

None 

68 

200  c.c.  i|%  CUSO4.5H2O  and 

2 % sulphuric  acid 

O.6359 

O.5282 

O.IO77 

0.1033 

2% 

69 

200  c.c.  2 % sulphuric  acid 

None 

None 

None 

0.0532 

2% 

The  enrichment  products  partly  replacing  the  sphalerite  were 
removed  with  a 1 per  cent,  solution  of  potassium  cyanide  and 
analyzed : 

TABLE  XLI. 

Analyses  of  the  Enrichment  Products. 


Cu  in  KCN, 

S in  KCN, 

Total, 

Per  Cent. 

Per  Cent. 

Per  Cent. 

Exp. 

g. 

g- 

g- 

of  Cu. 

of  CU2S. 

of  CuS. 

67  (no  acid) 

0.0572 

O.0234 

O.0806 

71.0 

34 

66 

68  (acid) 

0.0695 

O.0324 

0.I0I9 

68.2 

13 

87 

These  analyses  show  that  when  sulphuric  acid  is  present  as  an 
initial  constituent  of  the  solution,  more  cupric  sulphide  is  formed 
than  when  no  acid  is  present  initially.  The  analysis  of  the  solu- 
tion in  experiment  69  showed  that  the  acid  had  attacked  the 
sphalerite,  carrying  zinc  into  solution  and  liberating  hydrogen 
sulphide.  No  doubt  this  reaction  also  takes  place  in  experiment 
68,  but  the  hydrogen  sulphide  is  used  up  in  precipitating  cupric 
sulphide.94  Thus  the  acid  should  accelerate  the  reaction  repre- 
sented by  the  equation  ZnS  + CuS04  = CuS  -f-  ZnS04. 

Inasmuch  as  the  reaction  represented  by  equation  ( 1 1 ) is  ac- 
celerated by  sulphuric  acid,  we  should  find  more  zinc  in  solution 
in  experiment  68  than  in  experiment  67.  An  examination  of  the 
analyses  of  the  solutions  will  show  that  such  was  the  case,  but  it 
is  rather  interesting  to  note  that  the  difference  between  the 
amounts  of  zinc  thus  found  is  not  as  great  as  the  amount  of  zinc 
found  when  only  acid  was  used.  This  is  probably  due  to  the  fact 
94  See  Posnjak,  Allen  and  Merwin,  this  journal,  X.,  528,  1915. 


492 


E.  G.  Z1ES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 


that  the  sphalerite  on  becoming  coated  with  the  enrichment  prod- 
ucts is  largely  protected  from  attack.  If  the  enrichment  prod- 
ucts when  formed  had  remained  distinct  from  the  sphalerite,  and 
thus  permitted  a fresh  surface  of  the  sphalerite  to  be  exposed  to 
the  action  of  the  solution,  the  difference  between  the  amounts 
of  zinc  found  in  experiments  67  and  68  should  be  at  least  as  large 
as  the  amount  of  zinc  found  when  only  acid  is  used.  The  en- 
richment products,  however,  adhere  firmly,  thus  exhibiting  the 
characteristic  feature  of  natural  enrichment  products. 

Discussion. — The  evidence  brought  out  by  our  experimental 
work  on  the  reaction  between  sphalerite  and  cupric  sulphate 
shows  that  sphalerite  at  all  temperatures  first  reacts  with  cupric 
sulphate  to  form  cupric  sulphide,  and  that  this  in  turn  is  further 
attacked  by  the  cupric  sulphate,  yielding  cuprous  sulphide  and 
sulphuric  acid.  Finally,  sulphuric  acid  accelerates  the  reaction 
in  which  cupric  sulphide  is  formed. 

II.  THE  REACTION  BETWEEN  GALENA  (PbS)  AND  CUPRIC  SUL- 
PHATE AT  ORDINARY  TEMPERATURES. 

The  preliminary  experiments  carried  out  on  this  sulphide 
showed  that  accurate  deductions  could  be  made  from  work  done 
at  ordinary  temperatures,  inasmuch  as  galena  exhibited  a sur- 
prising activity  towards  cupric  sulphate  solutions  under  these 
conditions.  As  a matter  of  fact  the  work  at  elevated  tempera- 
tures in  the  bombs  was  not  as  satisfactory  as  that  done  at  the 
ordinary  temperatures  where  the  shaking  machine  could  be  used ; 
for  the  insoluble  lead  sulphate  which  was  formed  clogged  up  the 
voids  between  the  grains  of  the  galena  and  practically  stopped 
the  reaction.  This  difficulty  was  avoided  at  ordinary  tempera- 
tures where  the  contents  of  the  flasks  were  agitated.  The  galena 
used  in  the  experiments  came  from  the  Mississippi  valley  region 
in  Wisconsin.  It  was  carefully  selected  from  several  pounds  of 
material  and  analyzed  as  follows : 


Material. 

Pb. 

S. 

Cu. 

Fe. 

Zn. 

SiO*.  i PbS04. 

Mn. 

Galena  from  Miss. 
Valley 

86.53 

1 86.58 

I3-3I 

13.41 

Trace95 

Trace95 

Trace 

Trace95|  00.06 

None 

Calculated  for  PbS . . . 

95  Less  than  0.01  per  cent. 


SECONDARY  COPPER  SULPHIDE  'ENRICHMENT.  493 


The  analysis  shows  that  no  manganese  was  contained  in  the 
galena,  hence  the  surprising  activity  mentioned  above  can  not  be 
due  to  the  presence  of  alabandite  (MnS).96 

In  the  first  experiment  tabulated  below,  the  galena  was  ground 
to  pass  through  a ioo-mesh  bolting  cloth;  no  closer  sizing  was 
resorted  to.  In  the  second  experiment,  the  material  was  sized 
between  125  and  200  mesh. 

In  these  experiments  the  copper  was  deposited  on  the  galena 
as  a firmly  adhering,  bluish  coating  of  copper  sulphide. 

TABLE  XLII. 

Galena  and  Cupric  Sulphate. 

Initial  Conditions. 


Weight, 

g- 

Solution. 

| 

Exp. 

Material. 

Quantity. 

Copper, 
Initial,  g. 

Temp. 

70 

Galena,  Miss.  Valley, 
100  mesh  and  finer. 

3.000 

ij%  CUSO4.5H2O  in 

200  C.C. 

O.639O 

35  ± 5 

71 

Galena,  Miss.  Valley, 
125-200  mesh. 

9.048 

i\%  CUSO4.5H2O  in 

400  C.C. 

I.2718 

35  ± 5 

The  analyses  of  the  resulting  solutions  and  of  the  lead  sulphate 
are  shown  below : 


TABLE  XLIII. 


Analyses  of  Solutions  and  of  Lead  Sulphate. 


Exp. 

Duration,  * 
Months. 

Copper,  g. 

Acid, 

H2S04, 

g- 

PbS04. 

Equiv.  Pb. 

Initial. 

Final. 

Deposited. 

70 

597 

0.6390 

O.2540 

0.3850 

0.065 

1.6936 

1. 1570 

71 

2 

I.2718 

1.1652 

O.IO66 

0.020 

— 

— 

The  surface  of  galena  exposed  in  70  was  determined  approxi- 
mately and  found  to  be  about  the  same  as  that  exposed  in  71. 98 
This  comparison  is,  however,  only  approximate.  The  difference 
06  See  G.  S.  Nishihara,  Econ.  Geol.,  IX.,  743,  1914. 

97  Remained  in  shaking  machine  for  a period  of  one  month,  but  contents 
of  bottle  were  not  analyzed  until  four  months  later. 

98  Determined  on  the  basis  of  microscopic  examination. 


494 


E.  G.  ZIES,  Et  T.  ALLEN  AND  H.  E.  MERWIN. 


in  amount  of  copper  deposited  in  the  two  experiments  is  prob- 
ably largely  due  to  the  difference  in  duration  of  the  experiments. 

Methods  of  Analysis. — The  results  shown  in  Table  XLIII  were 
secured  as  follows : After  filtering  off  the  solution  and  drying 
the  residue  in  a vacuum  desiccator  over  sulphuric  acid,  the  copper 
in  solution  was  determined  as  usual  by  electrolysis  after  deter- 
mining the  sulphuric  acid  by  titration ; when  such  small  quantities 
of  acid  as  represented  above  are  to  be  determined  by  titration  in 
the  presence  of  copper  sulphate,  the  difficulty  involved  in  obtain- 
ing a sharp  end  point  renders  an  exact  determination  impossible ; 
the  figures  are  therefore  only  approximate.  The  lead  sulphate 
formed  during  the  experiments  was  separated  from  the  galena 
and  its  adherent  coating  of  copper  sulphide  by  taking  advantage 
of  the  solubility  of  lead  sulphate  in  ammonium  acetate;  galena  is 
insoluble  in  this  reagent;  the  lead  sulphate  is  reprecipitated  with 
dilute  sulphuric  acid  and  determined  in  the  usual  manner.  The 
amount  of  lead  sulphate  which  remains  in  solution  with  the 
cupric  sulphate  is  negligible."  The  firmly  adhering  sulphides  of 
copper  were  removed  from  the  unchanged  galena  by  a 2 per  cent, 
solution  of  KCN  and  analyzed  as  usual.  A blank  test  proved 
that  galena  is  unaffected  by  this  solution. 

It  is  impossible  to  determine  both  lead  sulphate  and  the  cop- 
per sulphides  in  the  same  sample,  for  the  ammonium  acetate  used 
to  dissolve  the  former  attacks  the  sulphides  of  copper;  and  fur- 
thermore the  potassium  cyanide  which  is  used  to  dissolve  the 
copper  sulphides  changes  some  of  the  lead  into  sulphide.  Each 
of  the  substances  may  however  be  determined  separately,  for  pure 
lead  sulphate  may  be  precipitated  from  the  ammonium  acetate 
solution,  and  though  a part  of  the  sulphur  to  be  determined  in 
the  potassium  cyanide  solution  is  precipitated  as  lead  sulphide, 
an  equivalent  of  sulphate  passes  into  solution  at  the  same  time 
thus:  K2S  + PbS04  = PbS  + K2S04.  Besides,  lead  sulphate 
itself  is  not  appreciably  soluble  in  potassium  cyanide. 

The  deposit  on  the  galena  in  the  second  experiment  therefore 
was  analyzed  as  usual. 

r'°  That  is,  as  compared  with  the  total  lead  sulphate  deposited. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT . 495 


Analysis  of  Enrichment  Products. 


Exp. 

Cu  in  KCN,  g. 

S in  KCN,  g. 

Total,  g. 

Per  Cent, 
of  Cu. 

Per  Cent 
of  CU2S. 

Per  Cent, 
of  CuS. 

71 

O.O428 

O.O197 

0.0625 

68.5 

15  ± 5 

85 

It  is  evident  that  the  reaction  between  galena  and  cupric  sul- 
phate can  not  be  expressed  by  the  following  simple  equation 
alone : 

PbS  + CuS04  = CuS  + PbS04.  ( 12) 

The  presence  of  the  cuprous  sulphide  in  the  enrichment  products 
is  further  shown  by  the  presence  of  sulphuric  acid  in  the  solu- 
tion, this  product  being  readily  accounted  for  by  the  further 
change  of  covellite  with  copper  sulphate  (see  equation  (i)).  If 
this  view  is  correct  we  should  be  able  to  calculate  the  quantities  of 
cupric  and  cuprous  sulphide  from  the  quantity  of  acid  found  in 
solution  and  the  total  copper  precipitated.  The  calculation  is  not 
very  exact  because  the  acid  can  be  determined  only  approxi- 
mately. The  determination  tends  to  be  too  high  on  account  of 
the  disturbing  influence  of  the  cupric  sulphate  on  the  end  point. 
The  calculation  gives  79  per  cent,  of  cupric  sulphide  and  21  per 
cent,  of  cuprous  sulphide  which  agree  with  the  direct  determina- 
tions as  closely  as  can  be  expected. 

In  experiment  70,  on  page  494,  the  presence  of  sulphuric  acid 
and  the  molecular  ratio  of  copper  lost  by  the  solution  to  the  lead 
deposited,  again  indicate  the  presence  of  both  cupric  and  cuprous 
sulphide : 


Exp. 

I Copper  Lost  by  Solution,  g. 

Lead  Deposited,  g. 

T?  • CU 

RauoPb' 

70 

0.3850 

I.I57 

I.O84 

Here  the  amounts  of  copper  and  lead  are  large  and  can  be  deter- 
mined very  accurately.  If  all  the  copper  were  present  as  cupric 
sulphide,  the  above  ratio  should  be  unity  as  demanded  by  equa- 
tion (12). 


496  E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 

Assuming  that  the  acid  formed  during  the  experiment  was  due 
to  the  formation  of  cuprous  sulphide  according  to  equation  (i), 
and  knowing  the  amount  of  the  acid,  we  can  determine  the 
amount  of  copper  which  reacted  with  the  cupric  sulphide  to 
form  cuprous  sulphide.  Making  this  calculation  we  find  that 
0.0316  g.  Cu  reacted  in  this  manner.  We  have  just  seen  that 
0.3850  g-  of  copper  took  part  in  the  total  reaction.  Deducting 
the  former  from  the  latter  we  have  0.3534  g.  of  copper  which 
took  part  in  the  direct  reaction  between  cupric  sulphate  and 
galena.  The  molecular  ratio  of  this  copper  to  the  lead  deposited 
as  lead  sulphate  is  Cu/Bb=i.oo.  This  is  the  ratio  demanded 
by  equation  (1 2). 

In  consequence  of  all  this  evidence  on  the  action  of  cupric  sul- 
phate on  galena,  we  feel  confident  in  stating  that  when  these  sub- 
stances react,  cupric  sulphide  is  first  formed  and  that  this  cupric 
sulphide,  even  at  ordinary  temperatures,  is  further  attacked  by 
the  cupric  sulphate,  yielding  cuprous  sulphide. 

V.  RELATIVE  REACTIVITIES  OF  THE  SULPHIDES  TOWARDS 
CUPRIC  SULPHATE,  AT  40°. 

It  is  obviously  very  desirable  to  know  the  relative  rate  of 
copper  enrichment  and  also  the  relative  amounts  of  mineral 
changed  when  the  sulphides  react  with  cupric  sulphate.  After 
carrying  out  several  preliminary  experiments,  we  became  con- 
vinced, however,  that  it  is  impossible  to  obtain  accurate  data  on 
this  problem. 

First:  In  making  such  a comparison  it  is,  of  course,  necessary 
to  compare  equal  surfaces  of  the  sulphides.  It  is  impossible  to 
do  this  since,  owing  to  the  presence  of  fissures  and  cleavage 
cracks,  the  apparent  surface  is  not  the  true  surface  exposed  to  the 
action  of  the  solution,  and  each  of  the  minerals  is  likely  to  differ 
in  amount  of  Assuring  and  cleavage. 

Second:  We  have  shown  in  the  experimental  work  just  dis- 
cussed that  the  copper  sulphide  enrichment  products  envelop  and 
adhere  very  firmly  to  the  grain  of  the  sulphide  which  precipitated 
them.  It  is  evident,  therefore,  that  we  can  not  determine  the  rate 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  497 


of  the  reaction  since  the  coating  will  prevent  ready  access  of  the 
solution  and  retard  the  reaction  more  and  more  as  its  thickness 
increases. 

Third:  Then,  too,  the  determination  of  the  rates  of  enrichment 
is  further  complicated  by  the  fact  that  at  ordinary  temperatures, 
within  the  limit  of  the  time  which  is  feasible  for  carrying  out 
these  experiments,  the  sulphides  do  not  alter  to  an  enrichment 
product  common  to  all  of  them.  Thus  when  chalcopyrite,  bor- 
nite,  sphalerite  and  galena  are  enriched,  both  covellite  and  chal- 
cocite  are  found  in  the  enrichment  product,  and  the  relative 
amounts  of  the  two  copper  sulphides  differ  for  each  sulphide 
used.  Pyrrhotite  alters  first  to  chalcopyrite  but  the  enrichment 
products  of  chalcopyrite  are  also  present. 

Fourth:  The  determination  of  the  rate  of  alteration  of  the 
sulphides  to  chalcocite  is  without  doubt  also  very  desirable  and 
would  form  an  excellent  basis  for  comparing  the  relative  reac- 
tivities of  the  sulphides.  In  order  to  approach  natural  enrich- 
ment conditions,  experiments  directed  along  this  line  must  be 
carried  out  at  ordinary  temperatures  but  at  such  temperatures  the 
alteration  of  the  sulphides  to  chalcocite  is  exceedingly  slow  and  at 
present  beyond  the  possibilities  of  laboratory  study.  At  elevated 
temperatures,  200°  for  instance,  the  experimental  work  has 
shown  that  alteration  to  chalcocite  takes  place  far  more  readily 
but  we  can  by  no  means  be  certain  that  relative  rates  determined 
at  such  a temperature  are  applicable  at  lower  temperatures. 

This  being  the  case,  a comparison  of  the  rates  of  enrichment 
must  be  confined  at  present  to  a comparison  of  the  amounts  of 
copper  (as  sulphide)  precipitated  by  the  sulphides  and  the  extent 
of  the  alteration  of  these  sulphides. 

All  of  the  foregoing  statements  are  of  course  only  applicable 
when  cupric  sulphate  is  the  enriching  agent.  Experiments  car- 
ried out  by  Winchell  and  Spencer  on  pyrite  and  our  own  experi- 
ments on  covellite  strongly  indicate  that  enrichment  will  proceed 
faster  when  cuprous  sulphate  is  the  enriching  agent.100 

It  must  be  remembered,  however,  that  these  same  principles 

100  See  page  428. 


498 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


are  applicable  to  natural  conditions.  The  relative  values  shown 
in  the  table  below  furnis'h  at  least  an  indication  of  what  to  expect 
as  to  the  rapidity  with  which  the  various  sulphides  are  altered 
and  enriched  in  nature. 

Experimental  Work. — In  order  to  obtain  approximately  equal 
surfaces,  first,  the  sulphides  were  carefully  sized  between  125  and 
200  mesh,101  and  second,  equal  volumes,  based  on  the  densities  of 
the  sulphides,  were  used.  The  sulphides  were  exposed,  at  40°,  to 
the  action  of  a 1%  per  cent,  solution  of  CuS04.5H20,  for  a 
period  of  two  months.  The  contents  of  the  flask  were  gently 
agitated102  throughout  this  period. 


TABLE  XLIV. 


I. 

II. 

III. 

IV. 

V. 

VI.  ‘ 

VII. 

VIII. 

IX. 

H3  . 

II 

s 

73 

c3 

§ 

Wt.  of  Sulphide  Use 
Based  on  Equal  Vol 
umes  (g.). 

Copper  Deposited 
(&■). 

Metal  Derived  from 
Sulphide  (g.)-104 

Wt.  of  Mineral 
Altered  (g.)jCalcu- 
lated  from  IV. 

Mols.  ot  Mineral 
Altered  Based  on 
IV. 

Relative  Mols  of 
Mineral  Altered 
CusFeSi  =1. 

Relative  Wt.  of  Min- 

eral Altered,  CuFeS2  = 
1 Based  on  V. 

oS 

"2 

0 5 11 

>73  tn 

‘•2  h 3 

Galena,  Miss. 

Val.,  Wise.  . . 
Pyrrhotite, 

9.048 

0.1070 

0.3258  Pb 

0.3762  PbS 

0.00157 

4-5 

5-5 

3-0 

Copper  Mt., 

A In  qItq 

5-593 

O.0896 

106 

1 



Sphalerite, 
Sonora,  Mex. 
Pyrrhotite, 

4.881 

O.0616 

0.0528  Zn 

0.0787  ZnS 

0.00081 

2.3 

1.1 

1. 1 

Orange  Co., 
V^r 

5-593 

O.0564 

106 

Chalcopyrite, 

Evora, 
Portugal .... 

5.000 

0.0308 

0.0210  Fe 

0.0691  CuFeS2 

O.OOO38 

1. 1 

1.0 

1.0 

Bornite, 

Superior, 

Ariz 

5-952 

0.0223 

! 0.0197  Fe 

0.1771  Cu5FeS4 

O.OOO35 

1 

2-5 

2.1 

101  See  page  410.  Also,  up  to  the  present  we  have  not  been  able  to  obtain 
bolting  cloth  which  would  permit  even  approximately  accurate  sizing  of  finer 
material. 

102  Violent  agitation,  of  course,  would  have  broken  up  the  grains  by  attri- 
tion, thus  materially  changing  the  surface. 

103  The  analyses  of  these  sulphides  are  given  under  the  description  of  the 
previous  experimental  work  on  the  reactions  between  them  and  copper 
sulphate. 

104  This  column  shows  the  amount  of  lead  deposited  as  sulphate  and  also 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT. 


499 


The  action  at  40 ° of  cupric  sulphate  on  both  Elba  pyrite  and 
Butte  covellite,  sized  in  the  manner  indicated,  is  so  slight,  and 
the  experimental  errors  are  so  large  that  the  values  obtained  were 
not  included  in  the  table.  When  a method  is  developed  for  accu- 
rately sizing  material  ground  finer  than  that  used  in  the  experi- 
ments, it  will  be  possible  to  assign  values  to  these  substances. 

The  table  brings  out  the  following  interesting  features : Galena 
is  very  much  more  reactive  towards  cupric  sulphate  than  any 
of  the  other  sulphides  considered  in  this  paper,  no  matter  on 
what  basis  the  comparison  is  made;  the  pyrrhotites  differ  mark- 
edly in  their  precipitating  power;  bornite  follows  chalcopyrite  in 
precipitating  power,  but  follows  galena  in  volume  of  mineral 
altered. 

VI.  SUMMARY. 

1.  The  reactions  of  a number  of  natural  sulphides  with  copper 
sulphate  solutions  have  been  quantitatively  investigated.  Atten- 
tion has  been  confined  to  the  following:  chalcocite  (Cu2S), 
covellite  (CuS),  bornite  (Cu5FeS4),  chalcopyrite  (CuFeS2), 
pyrrhotite,  pyrite  (FeS2),  sphalerite  (ZnS)  and  galena  (PbS). 
In  all  cases  a copper  enrichment  product  is  formed,  either  a 
sulphide  which  varies  with  the  conditions,  or  as  a special  case, 
metallic  copper  and  cuprite.  In  all  cases  also  the  sulphate  of  the 
metal  contained  in  the  original  sulphide  is  formed  and  usually 
sulphuric  acid  as  well.  This  acid  is  derived  from  the  oxidation 
of  the  sulphur  in  the  sulphide  with  cupric  sulphate.  In  these 
reactions  cupric  sulphate  plays  the  role  of  an  oxidizing  agent; 
not  only  at  elevated  temperatures  but  at  lower  temperatures  as 
well. 

2.  The  sulphide  enrichment  products  are  crystalline  and  all 
adhere  firmly  to  the  altered  sulphide,  as  in  nature.  When  cupric 
sulphate  is  the  enriching  agent,  pyrite  alters  to  covellite  and  chal- 

the  amounts  of  zinc  and  iron  found  in  solution  as  sulphates.  They  are  the 
total  amounts  which  took  part  in  the  reactions  between  the  sulphides  from 
which  they  were  derived  and  cupric  sulphate. 

105  Based  on  V and  the  densities  of  the  sulphide. 

108  The  amount  of  iron  in  solution  is  not  the  total  amount  of  iron  which 
took  part  in  the  reaction  between  pyrrhotite  and  cupric  sulphate.  See  page 
459  for  further  explanation. 


500  E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  MERWIN. 

I 

cocite.  It  has  been  shown  that  the  alteration  to  chalcocite  is  rep- 
resented by  the  following  equation: 

5FeS2  + 14G1SO4  + i2H20=^7Cu2S  +;5FeS04  + I2H2S04, 

and  that  in  the  alteration  to  covellite  the  following  equation  in 
all  probability  represents  the  reaction: 

4FeS2  -\-  yCuS04  — (-  4FT20 7^uS  d-  4FcS04  -f-  4H2S04. 

The  evidence  is  good  that  this  reaction  is  involved  when  pyrite 
alters  to  chalcocite.  Pyrrhotite  alters  to  chalcopyrite  and  very 
probably  to  bornite.  The  reaction  can  not  at  present  be  satis- 
factorily worked  out  on  a quantitative  basis  owing  to  the  fact  that 
pyrrhotite  varies  in  composition  and  is  attacked  by  one  of  the 
reaction  products,  namely  sulphuric  acid.  Chalcopyrite  alters  to 
covellite  and  chalcocite.  The  reaction  between  chalcopyrite  and 
cupric  sulphate  to  form  chalcocite  has  been  shown  to  be  repre- 
sented by  the  equation: 

5CuFeS2  + nCuS04  -H,8H20  = 8Cu2S  + 5FeS04  + 8H2S04. 

When  chalcopyrite  alters  to  covellite  the  experiments  point 
strongly  to  the  reaction  represented  by  the  following  equation : 

CuFeS2  + CuS04  = 2CuS  + FeS04, 

and  also  indicate  that  this  reaction  is  involved  in  the  alteration 
to  chalcocite.  Bornite  alters  to  chalcocite  as  follows : 

5Cu5FeS4+iiCuS04+8H20=i8Cu2S+5FeS04+8H2S04. 

It  has  also  been  shown  that  bornite  may  alter  to  covellite  and  chal- 
cocite thus : 

Cu5FeS4  + CuS04  = 2Cu2S  + 2Q1S  + FeS04. 


Covellite  alters  to  chalcocite  as  follows : 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT.  50 


5C11S  + 3C11SO4  + 4H20  = 4Cu2S  + 4H2SO4. 

The  experiments  also  furnish  evidence  that  this  reaction  proceeds 
in  two  stages  thus : 

CuS  — f-  7CUSO4  -f-  4H2O  = 4CU2SO4  — |-  4^2^04, 

CuS  -f-  Cu2S04  = Cu2S  -f-  CuS04. 

It  is  very  probable  that  these  two  reactions  are  involved  when  the 
sulphides  discussed  in  this  paper  react  with  cupric  sulphate  to 
form  chalcocite.  Sphalerite  and  galena  alter  first  to  covellite  and 
subsequently  to  chalcocite : 

ZnS  + CuS04  — CuS  + ZnS04, 

5CuS  -f-  3CUSO4  + 4H2O  ==  4Cu2S  -f-  4H2SO4. 

PbS  + CuS04  ==  CuS  + PbS04, 

5CuS  -f-  3CUSO4  -f-  4H20  ==  4Cu2S  + 4H2S04. 

3.  The  order  of  stability  of  the  sulphide  enrichment  products 
toward  cupric  sulphate  solutions  is : chalcopyrite,  covellite,  chap 
cocite ; each  of  them  changing  into  the  succeeding  sulphide  by  the 
further  action  of  cupric  sulphate.  Chalcocite  is  by  far  the  most 
stable  sulphide  of  all,  under  these  conditions,  but  it  may  finally 
be  converted  into  metallic  copper  and  sulphuric  acid,  though  very 
slowly  indeed  even  at  200°.  The  most  favorable  conditions 
which  have  been  observed  for  the  formation  of  the  intermediate 
products,  chalcopyrite  and  covellite,  are  the  exposure  of  a large 
surface  of  the  reacting  sulphide  to  the  action  of  a comparatively 
dilute  solution  of  copper  sulphate. 

4.  All  these  reaction's  have  been  studied  at  several  tempera- 
tures ranging  from  200°  down  to  30°.  In  the  main  the  rate 
rather  than  the  nature  of  the  reaction  is  changed  by  raising  the 
temperature,  but  there  are  a number  of  secondary  reactions,  slight 
or  negligible  at  low  temperatures,  which  become  pronounced  at 


502 


E.  G.  ZIES,  E.  T.  ALLEN  AND  H.  E.  M ERWIN. 


higher  temperatures.  Thus  ferrous  sulphate  is  partly  changed 
into  ferric  sulphate  by  cupric  sulphate : 

2Cife04  + 2FeS04  ^ Cu2S04  4"  Fe2(S04)3. 

At  elevated  temperatures,  hydrolysis  generally  conditions  the  for- 
mation of  considerable  hematite,  cuprite  and  metallic  copper  from 
the  two  primary  products  of  the  above  reaction.  To  what  extent 
the  metallic  copper  and  cuprite  sometimes  found  in  the  natural 
enrichment  zone  are  derived  from  the  hydrolysis  of  cuprous  sul- 
phate is  not  clear  from  these  experiments. 

5.  The  results  of  qualitative  experiments  indicate  that  enrich- 
ment proceeds  faster  in  the  presence  of  cuprous  sulphate  than  in 
the  presence  of  cupric  sulphate. 

6.  The  influence  of  sulphuric  acid  on  the  enrichment  reactions 
has  been  studied.  The  enrichment  of  chalcopyrite  and  pyrite 
in  our  experiments  has  been  retarded  by  an  increase  in  the  con- 
centration of  sulphuric  acid.  The  explanation  for  this  is  found 
in  the  fact  that  hydrolysis  of  the  ferric  sulphate,  formed  as  we 
have  stated  above  (paragraph  4 of  summary),  is  either  hindered 
or  prevented,  and  thus  the  influence  of  the  cuprous  sulphate 
formed  from  cupric  sulphate  by  the  reducing  action  of  ferrous 
sulphate  is  held  back.  The  result  is  that  the  formation  of  cuprous 
sulphate  is  limited,  and  since  the  rate  of  reaction  of  cuprous  sul- 
phate on  the  sulphides  is  much  faster  than  that  of  cupric  sul- 
phate, enrichment  itself  is  retarded.  The  enrichment  of  galena, 
sphalerite,  pyrrhotite,  and  bornite  is  accelerated  by  sulphuric  acid, 
for  the  “solubility”  of  these  sulphides  is  thus  materially  in- 
creased. Chalcopyrite  is  one  of  the  products  at  higher  tempera- 
tures between  bornite  and  2 per  cent,  sulphuric  acid  alone. 

7.  The  influence  of  ferrous  sulphate  on  the  enrichment  reac- 
tions has  also  been  studied  to  some  extent.  The  first  effect  is  to 
increase  the  rate  by  increasing  the  quantity  of  cuprous  sulphate 
in  solution,  and  cuprous  sulphate  is  more  rapid  than  cupric  in  its 
action  on  the  sulphides.  However,  the  effect  is  soon  lost  unless 
the  ferric  iron  formed  is  removed  from  solution. 


SECONDARY  COPPER  SULPHIDE  ENRICHMENT . 503 


It  may  be  stated  here  that  a reversal  of  the  principal  enrich- 
ment reactions,  such  for  example  as 

f 

5FeS2+i4CuS04+^H20=7Cu2S+5FeS04+\H2S04, 

has  not  been  realized  experimentally.  The  attempt  to  intro- 
duce iron  into  bornite  by  allowing  the  sulphide  to  react  with  fer- 
rous sulphate  alone  has  not  met  with  success. 

Geophysical  Laboratory, 

Carnegie  Institution  of  Washington, 

Washington,  D.  C. 


